WASHINGTON -- As Election Day arrives, Democrats are in a strong position to maintain their majority in the Senate. But for much of 2012, Democratic control of the chamber looked unlikely.
Republicans need to win a net gain of four seats to achieve a majority, which once seemed achievable. First, there were retirements of Democratic incumbents in conservative and competitive states, including Nebraska, North Dakota, Virginia, Wisconsin, and New Mexico.
Additionally, Democratic incumbents looked vulnerable in Missouri, Montana, Ohio, and Florida for most of the year. Meanwhile, it seemed that the only seat that Republicans would have to actively defend was Massachusetts.
However, strong Democratic candidates in North Dakota and Arizona put two seats in play that had been firmly in the Republican column, while weak Republican candidates in Missouri, Indiana, Florida, Ohio, and Michigan may have cost the GOP some winnable races in those states.
Particularly troubling to Republicans should be the campaigns of their ultra-conservative nominees in Missouri and Indiana. They both defeated more moderate, electable opponents in primaries and made controversial rape-related remarks that stole the national media spotlight. Now both are underdogs.
If both candidates lose, that would mark five potential Senate seats that Republicans have lost in the last two years due the strong conservative ideology of nominees. In 2010, Tea Party-backed candidates in Delaware, Nevada and Colorado defeated more moderate candidates in the primaries, only to lose in the general election.
Now there are four Republican seats in play and Democrats have solidified their leads in a number of races. Thus, the GOP has a tough -- but certainly not impossible -- path to a Senate majority.
The combination of returning senators and candidates currently leading by at least three points in the current HuffPost Pollster estimates in 2012 contests would give the Democrats 50 seats. Additionally, one independent candidate that is likely to caucus with the Democrats continues to lead in polls in Maine, which would give them 51 seats -- just enough for an outright majority.
While holding the seats that are leaning in their direction, the GOP's most likely path to a majority is winning the four races currently rated as "toss-ups," in which neither candidate leads by more than a three-point margin in the HuffPost Pollster estimate, as well as winning one seat that is currently rated as "leaning Democratic" (or convincing independent Angus King of Maine to caucus with them).
Either scenario would result in a 50-50 tie in the Senate, which would amount to a majority if Mitt Romney is elected president, with Paul Ryan casting the tie-breaking vote as vice president. They would need to win an additional Democratic-leaning seat if President Barack Obama is reelected.
Meanwhile, Democrats could potentially maintain their current 53-47 majority, or even expand it by a few seats if they win a majority of the "toss-up" races.
Here are the breakdowns of the most competitive Senate races, in order of competitiveness, according to the final HuffPost Pollster estimates.
1.) North Dakota (D): Freshman Rep. Rick Berg (R-N.D.) was supposed to cruise to victory in this open seat race, but the strong candidacy of Heidi Heitkamp (D) turned this race into a unexpected toss-up. North Dakota's long tradition of ticket-splitting will be put to the test tomorrow since Heitkamp needs to outperform Obama by a significant amount to prevail. Heitkamp holds a statistically insignificant 0.3 point lead over Berg in the HuffPost Pollster estimate, which also shows that about 7 percent of voters are still undecided.
2.) Wisconsin (D): After narrowly trailing in the polls for most of September and October, former Gov. Tommy Thompson (R) finally seems to have regained his footing, once against turning the race against Rep. Tammy Baldwin (D-Wis.) to a pure "toss-up." Recent polls show that he continues to outperform Mitt Romney by about five points on average, so if Romney manages to keep it close, Thompson could pull off the win. He currently trails Baldwin by 0.8 points in the HuffPost Pollster estimate.
3.) Montana (D): Sen. Jon Tester (D-Mont.) and Rep. Denny Rehberg (R-Mont.) have been deadlocked in the polls for nearly two years, despite the barrage of ads from outside groups on both sides. While Tester currently has a 1.1 point lead in the HuffPost Pollster estimate, he must outperform Obama, who is expected to lose the state by as much as 10 points.
4.) Virginia (D): Former Gov. Tim Kaine (D) holds a 2.6 point advantage over former Sen. George Allen (R-Va.) in the HuffPost Pollster estimate of this race, just a bit larger than Obama's lead in the state. It seems unlikely that the Old Dominion will split its ticket this year, meaning that if Romney wins the state, Allen probably will, too.
5.) Nevada (R): Though Democrats hold a large edge in early voting in the Silver State, Sen. Dean Heller (R-Nev.) remains a modest favorite to win his first full term. However, a lopsided Obama victory, along with high Latino turnout, may be enough to carry Rep. Shelley Berkley (D-Nev.) to an unexpected victory. Heller leads Berkley by 3.6 points in the HuffPost Pollster estimate.
6.) Indiana (R): Richard Mourdock (R) and Rep. Joe Donnelly (D-Ind.) were running neck-and-neck all year in the polls until Mourdock said that pregnancies from rape were "something that God intended to happen" at an October debate. Every public poll released since then has found Donnelly ahead and he holds a 3.7 point lead in the HuffPost Pollster estimate. Mitt Romney is favored to comfortably carry this state, meaning that Donnelly will still need a lot of crossover votes to win. A potential wildcard is Libertarian candidate Andrew Horning, who has drawn as much as 6 percent support in recent polls.
7.) Arizona (R): Former U.S. Surgeon Gen. Richard Carmona's (D) high-profile candidacy turned this second-tier Senate race a nail-biter, but Rep. Jeff Flake (R-Ariz.) is in the stronger position heading into Election Day. While Flake leads Carmona by 3.8 points in the HuffPost Pollster estimate, nearly 7 percent of voters are still undecided.
8.) Massachusetts (R): Despite Sen. Scott Brown's (R-Mass.) posturing to the center, his party affiliation may very well cost him the Senate seat he won in a special election two years ago. The Republican brand is extremely unpopular in Massachusetts, expected to overwhelming vote against its former governor, Mitt Romney. Though Brown remains popular and a few polls still show him competitive, he currently trails Elizabeth Warren (D) by 4.5 in the HuffPost Pollster estimate, and Warren has passed the 50 percent threshold.
9.) Ohio (D): Sen. Sherrod Brown (D-Ohio) has withstood more than $26 million in attack ads from Republican-aligned outside groups, maintaining his polling lead over challenger Josh Mandel (R). Brown has managed to outperform Obama by a few points in the polls (partially due to Mandel's poor favorability rating), meaning he still may prevail in the event of a Romney victory in the Buckeye State.
10.) Pennsylvania (D): Sen. Bob Casey's (D-Pa.) sleepy reelection campaign brought this race some national attention as challenger Tom Smith (R) pulled within a few points of the incumbent in the polls. However, unless Mitt Romney's last-minute effort in the state brings him close to victory, it's hard to see how Smith comes out on top. Casey leads Smith by 5.1 points in the HuffPost Pollster estimate.
11.) Connecticut (D): For the second election cycle in a row, Linda McMahon (R) has spent tens of millions of dollars of her own money on her candidacy, but she is on track to come up short again this year. For most of August and September, she was running neck-and-neck with Rep. Chris Murphy (D-Conn.) in the polls, but she has not led in any publicly-released poll since Oct. 2 and trails Murphy by 6.2 points in the HuffPost Pollster estimate.
12.) New Mexico (D): This was once viewed as a top-tier race, but Rep. Martin Heinrich (D-N.M.) has emerged as the strong favorite due in large part to the state's wholesale shift to the left over the past several years. The National Republican Senatorial Committee canceled its ad reservations in this state in August and former Rep. Heather Wilson (R-N.M.) has failed to gain much traction in most polls -- she trails Heinrich by 7.8 points in the HuffPost Pollster estimate.
13.) Missouri (D): Rep. Todd Akin (R-Mo.) emerged from the August Republican primary as the favorite to unseat Sen. Claire McCaskill (D-Mo.), until he claimed that the female body can shut down pregnancies in cases of "legitimate rape." While McCaskill has pulled far ahead in most polls, she is still a very unpopular incumbent in a red state, so don't be surprised if this race turns out to be closer than indicated by the polls, which show him down by 8.7 points.
14.) Florida (D): Sen. Bill Nelson (D-Fla.) was once a top-tier target for Republicans, but has proved more resilient than anticipated. He has only trailed in one poll since August and is now outperforming Obama by nearly 10 points on average in the Sunshine State, attracting significant crossover support. Rep. Connie Mack (R-Fla.), who has not run a particularly strong campaign, has very few paths to victory even if Romney carries the state. He trails Nelson by 9.1 points in the HuffPost Pollster estimate.
HuffPost Pollster rates a race as a "toss-up" if the polling margin separating two candidates is less than three percentage points in the Pollster estimate and there have been at least five polls in that state in the last three weeks. A race is designated as "leaning" toward one party if a candidate is leading by three to six percentage points in that estimate. If a candidate is leading by more than six percentage points, it is rated as "strong" Democrat or Republican.
If there have been fewer than five polls in the last three weeks in any given race, composite ratings are used from three respected election handicappers: the Cook Political Report, the Rothenberg Political Report and Larry Sabato's Crystal Ball.
For more details and polls of all 33 Senate races, visit the HuffPost Pollster's Senate Outlook page.