Democrats currently hold a 53 to 47 majority in the Senate, including two independents that caucus with them. After gaining six seats in the 2010 midterm elections, Republicans need to net an additional four this year to reach a majority--or only three if Mitt Romney is elected president, since Paul Ryan would cast the tie-breaking Senate vote as vice president.

This year, Democrats have 23 seats up for reelection (including the two independents that caucus with them) while Republicans have only 10 seats up for reelection. According to the final HuffPost Pollster estimates, the combination of returning senators and candidates leading in the 2012 contests gives Democrats 50 seats and Republicans 45 seats. An independent candidate from Maine, who is expected to caucus with Democrats, continues to lead in the polls and there are four races rated as "toss-ups."

To win a majority, Republicans not only need to win those four "toss-up" races but also make at least one inroad into a race that currently "leans Democratic." Meanwhile, if Democrats win a majority of the "toss-up" races, they have an opportunity to expand their current majority.

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  • Arizona

    Last February, Sen. Jon Kyl (R-Ariz.) announced that he would not be seeking a fourth term in 2012. Rep. <strong>Jeff Flake</strong> (R-Ariz.), who emerged from a bruising, expensive Republican primary in August, will face <strong>Richard Carmona</strong> (D), a former surgeon general under President George W. Bush, in the general election. Flake and Carmona are deadlocked in the polls but many voters are still undecided, since the candidates were largely unknown to the broader Arizona electorate. Flake has faced criticism for his past <a href="">tenure as a lobbyist</a> for Namibian mining interests with ties to the Iranian government. Carmona, who has been distancing himself from President Barack Obama, is relying on heavy Latino turnout and his centrist political profile to offset the Republican Party's registration advantage in the state. Both national party committees are investing in this unexpectedly close race.

  • Connecticut

    Sen. Joe Lieberman (I-Conn.) announced in 2011 that he would be retiring from the Senate, creating an open seat in the Nutmeg State. Former World Wrestling Entertainment CEO <strong>Linda McMahon</strong> (R), who unsuccessfully ran for Senate in 2010 despite spending $40 million of her own money, is again the Republican nominee in 2012, and has run a much stronger campaign this time. Her opponent, Rep. <strong>Chris Murphy</strong> (D-Conn.), started off as the frontrunner after the primary despite his low name recognition, but the race tightened in August. Both campaigns have been launching prolonged negative ad offensives, and the DSCC has unexpectedly been forced to intervene on Murphy's behalf. McMahon has been trying to distance herself from the national Republican Party and position herself as a job-creating moderate.

  • Florida

    Despite middling job approval ratings, Sen. <strong>Bill Nelson</strong> (D-Fla.) is in a relatively strong position to win reelection. His opponent, Rep. <strong>Connie Mack IV</strong> (R-Fla.), was initially seen as a weak candidate due to his <a href="">questionable past</a> and <a href="">lackluster fundraising</a>, despite his family's fame in the state. Though polls pointed a close race over the summer, Mack has fallen behind Nelson in most polls since then. Several pro-Republican groups, such as the <a href="">U.S. Chamber of Commerce</a> and <a href="">Crossroads GPS</a> have aired ads attacking Nelson in an attempt to shrink his lead. However, time is running out and Nelson's lead may now be insurmountable.

  • Hawaii

    Sen. Daniel Akaka (D-Hawaii) announced he was stepping down in 2012, creating a rare open seat opportunity in the Aloha State. Though Republicans landed a strong recruit in former Gov. <strong>Linda Lingle</strong> (R), she remains the underdog in the general election against Rep. <strong>Mazie Hirono</strong> (D-Hawaii), especially with native son President Barack Obama at the top of the ticket. However, Lingle narrowly defeated Hirono in the 2002 governor's race and has raised more money than her opponent this year. This race has been sparsely polled so far, but Hirono has led by varying margins. Throughout the campaign, Lingle has been highlighting her <a href="">bipartisan credentials</a> as governor -- she even posted a picture of herself and President Obama on Facebook. Lingle must continue to campaign to the center to close the gap in this deeply Democratic state.

  • Indiana

    State Treasurer <strong>Richard Mourdock</strong> (R) handily defeated incumbent Sen. Richard Lugar (R-Ind.) in the May Republican primary by running far to his right, turning a safe Republican seat into a potential Democratic pick-up opportunity. Mourdock has a known disdain for compromise, and previously <a href="">questioned the constitutionality of Social Security and Medicare</a>, which he has since distanced himself from. Rep. <strong>Joe Donnelly</strong> (D-Ind.), the Democratic nominee, has sought to paint Mourdock as <a href="">too extreme for Indiana</a> while touting his own centrist record. Mourdock, meanwhile, has been <a href="">trying to tie Donnelly to President Barack Obama</a>, citing his vote in favor of Obamacare. Though Mourdock started off as the favorite, he took a plunge in the polls after he suggested that pregnancies from rape are "something that God intended to happen." The National Republican Senatorial Committee and the Democratic Senatorial Campaign Committee have both moved resources into the state in recent months.

  • Maine

    Sen. Olympia Snowe (R-Maine), one of the most moderate Republican senators, abruptly announced her retirement in February. <strong>Angus King</strong>, an independent former governor who <a href="">left office with high approval ratings</a> in 2002, emerged as the early favorite to replace her. He has typically led his two opponents, Secretary of State <strong>Charlie Summers</strong> (R) and state Sen. <strong>Cynthia Dill</strong> (D), by double-digit margins in most recent polls. Though King has not revealed which party he would caucus with if elected, his views seem to be more in line with Democrats, <a href="">especially on social issues</a>. The DSCC <a href="">has ignored Dill's request for an endorsement</a>, fearing a split in the Democratic vote, which may allow Summers to eke out a victory. The NRSC pulled its advertising from the state in October.

  • Massachusetts

    In the marquee Senate race of the cycle, Sen. <strong>Scott Brown</strong> (R-Mass.), who unexpectedly won Ted Kennedy's old Senate seat in a 2010 special election, is facing a much stronger candidate this time in <strong>Elizabeth Warren</strong> (D), a Harvard Law professor and special advisor for the Consumer Financial Protection Bureau. Polls have consistently shown an extremely tight race, while President Obama has consistently led Mitt Romney by double digits, which speaks to Brown's popularity in the state. Brown has largely been campaigning as an independent with bipartisan credentials, while Warren describes herself as a champion for the middle class, highlighting her efforts to make Wall Street and big banks more accountable to consumers. The race had largely been positive until Warren <a href="">launched an ad</a> in September going directly after Brown for supporting tax cuts for millionaires. The gloves finally came off at the three Senate debates, including a renewed interest in Warren's Native American heritage, but Warren goes into Election Day as the slight favorite.

  • Missouri

    Up until August, it looked as though incumbent Sen. <strong>Claire McCaskill</strong> (D-Mo.), a strong proponent of President Obama's agenda from a Republican-leaning state, was on pace to be a one-term senator. Democrats saw conservative Rep. <strong>Todd Akin</strong> (R-Mo.) as the weakest potential general election candidate and <a href="">spent $1.5 million on ads</a> calling him the "true conservative" in the race right before the Republican primary, hoping to boost his chances at winning the nomination. They got their wish when Akin won the primary in August, and a few weeks into the general election Akin <a href="">infamously said</a> that in cases of "legitimate rape," women can't get pregnant, causing a national uproar. Republicans of all stripes <a href="">urged Akin to drop out</a> of the race, but he stubbornly resisted and his name will appear on the ballot. The NRSC and American Crossroads have <a href="">pulled all funding</a> from the state, but Akin has started to go back on the offensive. While Akin now trails McCaskill in most polls, victory is not completely out of his grasp despite his <a href="">abysmal favorability rating</a>, which speaks to McCaskill's unpopularity among Missouri voters.

  • Montana

    Freshman Sen. <strong>Jon Tester</strong> (D-Mont.) is facing a tough challenge in 2012 from Rep. <strong>Denny Rehberg</strong> (R-Mont.), a top-tier Republican candidate who has represented the entire state since 2001. Unlike most Republicans running in red states, Rehberg has <a href="">highlighted his opposition</a> to Rep. Paul Ryan's (R-Wis.) budget proposal, which would overhaul Medicare. Both campaigns and a bevy of outside groups have run ads on behalf of their preferred candidate. Rehberg and Tester have been running neck-and-neck in the polls for nearly two years now, and this race remains a pure toss-up.

  • Nebraska

    When moderate Sen. Ben Nelson (D-Neb.) announced his retirement, Republicans saw it as a relatively easy opportunity to flip this seat in 2012. Democrats recruited former Sen. <strong>Bob Kerrey</strong> (D-Neb.) to run in Nelson's place. On the Republican side, state Sen. <strong>Deb Fischer</strong> (R) came from behind to defeat two better-known and better-financed opponents in the primary, helped by a <a href="">last-minute endorsement from former Alaska Gov. Sarah Palin (R)</a>. While Kerrey appeared to be making a comeback in October, he will likely fall short on Election Day, moving the Republican Party one seat closer to a Senate majority.

  • Nevada

    Amidst an ethics investigation, Sen. John Ensign (R-Nev.) <a href="">announced his resignation</a> from the Senate in April 2011. Then-Rep. <strong>Dean Heller</strong> (R-Nev.) was appointed to take his place and is now running as an incumbent senator in 2012. His opponent is Rep. Shelley Berkley (D-Nev.), <a href="">who is being investigated</a> by the House Ethics Committee for allegedly using the powers of her office to aid her husband's medical practice. Expecting a close race, the NRSC and DSCC have been flooding the airwaves with ads on behalf of their party's candidate. Most polls have shown Heller with a narrow edge over Berkley, but if Berkley can drive up Latino turnout and pull off the upset it would all but ensure that Democrats maintain a majority in the Senate.

  • New Mexico

    After Sen. Jeff Bingaman (D-N.M.) announced his retirement last February, Republicans were bullish on their chances to take over this seat. However, the Land of Enchantment has become much more Democratic over the last decade, and Rep. <strong>Martin Heinrich</strong> (D-N.M.), the Democratic nominee, has held a sturdy single-digit lead over former Rep. <strong>Heather Wilson</strong> (R-N.M.) in the polls. The NRSC tellingly <a href="">canceled its ads</a> in the state, though other Republican-aligned outside groups, such as <a href="">Crossroads GPS</a> and the <a href="">U.S. Chamber of Commerce</a>, have still been airing ads against Heinrich, particularly on energy issues. A coalition of environmentalist and conservationist groups have been <a href="">airing a barrage of negative ads</a> against Wilson. The shifting demographics of New Mexico may be too much for Wilson to overcome to pull off the upset.

  • North Dakota

    When Sen. Kent Conrad (D-N.D.) announced his retirement last year, most political observers expected this race to be an easy pickup for Republicans. Freshman Rep. <strong>Rick Berg</strong> (R-N.D.) announced he was running for the open seat and started off as the strong favorite to replace Conrad. However, Democrats recruited former state Attorney General <strong>Heidi Heitkamp</strong> (D) to run, and she is giving Berg a run for his money. All indications point toward a tight race: Heitkamp has been running neck-and-neck with Berg in the few polls that have been conducted and the NRSC <a href="">pulled money from two other races</a> and moved it to North Dakota. Republican-aligned outside groups <a href="">have run ads</a> attacking Heitkamp for supporting Obamacare, while Heitkamp has been running to the center, especially on energy issues. Although Heitkamp has proven to be a strong candidate, she must still outperform President Obama by a significant margin to prevail in North Dakota.

  • Ohio

    Many Republican-aligned outside groups have turned their attention to Ohio, where they have spent over $26 million dollars on ads in an attempt to sully the image of first-term Sen. <strong>Sherrod Brown</strong> (D-Ohio). While some polls have found that Brown's lead has shrunk since the ad barrage, his numbers have largely held up against his opponent, state Treasurer <strong>Josh Mandel</strong> (R), a 34-year-old Iraq war veteran and fundraising magnet with a <a href="">spotty ethical track record</a>. The auto bailout has been a big point of contention in the race. Brown voted for it while Mandel remains strongly against it -- he even called Brown "<a href="">un-American</a>" for supporting it -- arguing that it hurt the pensions of non-union employees. Even if Mitt Romney wins Ohio, it may not be enough to put Mandel over the top, given his <a href="">low favorability rating</a> in the state.

  • Pennsylvania

    Republicans failed to recruit a top-tier candidate to run against freshman Sen. <strong>Bob Casey</strong> (D-Pa.) in 2012, despite his mediocre approval ratings. Former coal company executive <strong>Tom Smith</strong> (R) emerged from the April Republican primary after spending <a href="">$4 million of his own money</a> on television advertising. However, Smith began to start closing in on Casey in the polls in September and October after loaning his campaign $10 million and flooding the state in ads, and has cut Casey's lead to just a few points.

  • Virginia

    After narrowly defeating incumbent <strong>George Allen</strong> (R-Va.) in 2006, Sen. Jim Webb (D-Va.) decided not to run for a second term in 2012, setting up a closely contested open seat race in an important swing state. Allen, who seems to have moved on from his 2006 “<a href="">Macaca" moment</a>, is running for his old seat. He is facing former Gov. <strong>Tim Kaine</strong> (D), who served as chairman of the Democratic National Committee under President Obama. The two men have been deadlocked in the polls for over a year and a half. Allen has repeatedly tried to associate Kaine with President Obama but Kaine appears to be embracing his ties to the president, which he expects will play especially well in Northern Virginia, while portraying himself as a centrist. Republicans will most likely have to win this race to have a shot at a majority in the Senate.

  • Wisconsin

    The retirement of four-term Sen. Herb Kohl (D-Wis.) turned a sure-fire Democratic hold into a possible pick-up opportunity for Republicans. Emboldened by Gov. Scott Walker's (R) recall victory in June and Rep. Paul Ryan's (R-Wis.) presence at the top of the ticket, the GOP thinks that <strong>Tommy Thompson</strong> (R), a former health and human services secretary and four-term governor, has a real shot to flip this seat. Most polls initially showed Thompson leading Rep. Tammy Baldwin (D-Wis.), who represents the liberal bastion of Madison, but the race is now almost dead-even heading into Election Day. Since the primary, <a href="">Baldwin's campaign</a> and several <a href="">Democratic groups</a> have launched an ad blitz against Thompson over his past as a Washington lobbyist, claiming that he is beholden to special interests. Thompson, who survived a bruising, expensive Republican primary, spent that time refilling his campaign coffers and was largely off the airwaves. Baldwin, who has a liberal voting record, is expected to underperform Obama in Wisconsin, which would make the math very difficult for her if Mitt Romney carries or comes close to winning the state.

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