BOSTON -- In the tight Massachusetts Senate race, GOP incumbent Scott Brown has spent weeks questioning Democratic rival Elizabeth Warren's claim of Native American heritage, while Warren portrays Brown as a darling of Wall Street.

The rhetoric is constant, sometimes caustic and all but invisible from the ad campaign waged on television.

That's because Brown and Warren signed a deal to discourage third party groups from running television, radio and online ads in Massachusetts. At this point, at least, their pleas seem to have been heard and two are leading by example.

In his TV ads, Brown shows himself as a cheerful bipartisan lawmaker, doting father and supportive husband. No mention is made of the issue that's become the near sole focus of his campaign: Warren's flummoxed handling of questions about her heritage.

Warren also has run a tougher campaign off-screen than on.

At campaign stops, she routinely refers to Brown as "Wall Street's favorite senator," a phrase that's absent from her TV ads, which aim to portray her as a fighter for the middle class.

The race is one of the country's most competitive as Democrats look to take back the seat long held by the late Sen. Edward M. Kennedy until Brown shocked the political world by winning it in 2010. Democrats hold a slight edge in the Senate and they see Massachusetts as their top chance to pick up a Republican-held seat to offset any losses they may incur elsewhere.

Yet, so far, Massachusetts voters have been spared the kind of corrosive ads that have flooded the airwaves in Nevada Virginia and other states with hotly contested Senate contests.

Brown and Warren can't stop outside groups from getting involved. But less than five months before the election, it seems those groups are taking the candidates' wishes to heart. If the groups stay on the sidelines, the candidates would have to use their own ads to attack each other, should they decide to go that route.

That poses a particular risk for Brown, who has tailored a good guy image in his ads. The most recent features his wife, former Boston television reporter Gail Huff. Brown also championed the ad deal, saying it would help elevate the political discourse.

John Baick, a professor of history at Western New England University, said that without the agreement, GOP-aligned political action committees already would be targeting Warren with harsh ads.

"I'm sure there are outside conservative groups who are frustrated," Baick said. "They could have an ad on the air in two minutes, but they can't. It's one of those nonissue issues that can play so well."

The dust-up over Warren's claim of Native American heritage surfaced at the end of April when The Boston Herald first reported that Harvard University, where Warren works as a law professor, had touted her as a Native American.

It was later revealed that Warren had listed herself in law school directories as having Native American heritage. She also identified her race as "white" on an employment record at the University of Texas and declined to apply for admission to Rutgers Law School under a program for minority students.

Warren has acknowledged telling Harvard and her previous employer, the University of Pennsylvania, of her Native American heritage, but only after she had been hired by both schools.

Warren, who grew up in Oklahoma, said she and her brothers were told by their parents that her mother was part Native American. She said she never sought proof of her ancestry and her campaign has been unable to verify the claim.

"My mom and dad were deeply in love," Warren said in recent interview with The Associated Press. "My father wanted to marry my mother, his parents objected, because she was part-Cherokee and part-Delaware."

Brown's campaign seized on the issue, arguing that Warren used the claim to gain an unfair advantage in hiring. Harvard Law School professor Charles Fried, a member of the committee that reviewed Warren for a job at the law school, said the subject of her Native American ancestry was never mentioned.

While Brown has steered clear of the issue in his television ads, he's raised it in news releases, during campaign stops and in interviews with the media.

"When you are running for elected office, especially high elected office, you have to pass a test and the test is about truthfulness and credibility and honesty and quite frankly she's failed that test, as evidenced by her claiming to be a Native American," Brown said during an interview Thursday on Fox News Channel's "Fox & Friends."

So far the agreement appears to be holding. Under the deal, a candidate who benefits from an advertisement from a third party group has to pay half the cost of the ad to a charity named by the other candidate.

To date, Brown has paid nearly $36,000 to Warren's charity. Warren hasn't had to write a check.

Both campaigns say they aren't backing away from the pledge.

An aide to Brown credited the pledge with keeping certain big-money political action committees and third-party groups off the Massachusetts airwaves.

"We're hopeful they will continue to stay out," said campaign spokesman Colin Reed. "In the end, voters will have a stark choice between Scott Brown, an independent thinker who is fighting for more jobs, low taxes and less debt, and Elizabeth Warren, a liberal extremist."

Doug Rubin, a senior adviser to Warren, said the campaign is also pleased with the deal.

"One of the benefits of the people's pledge for our campaign is that we have a very positive message about Elizabeth and her values and this allows the campaign to communicate that message directly to voters," he said.

While the pledge is helping hold the line against attack ads that could change the closer the race gets to Election Day, observers say.

Recent polls show a virtual dead heat between Warren and Brown with a small number of undecided voters who probably will decide the contest.

David Paleologos, director of the Suffolk University Political Research Center, said that if one candidate begins to slide in the polls, they may resort to the kind of attacks the pledge was meant to discourage.

"You've got a scenario where Brown has to, if he wants to advance the (Native American) issue, he has to advance it himself," Paleologos said. "The same thing with Elizabeth Warren. If she wants to engage Scott Brown on Wall Street she has to do it."

There are risks that any negative ads could create a backlash, he said.

"In that sense the pledge is a refreshing sign," he added. "It forces the candidates to take responsibility for the positions of their campaigns."

Here is some background on Elizabeth Warren:
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  • Introduces Financial Product Safety Commission

    Elizabeth Warren <a href="" target="_hplink">announced</a> a bill creating a Financial Product Safety Commission with House and Senate Democrats in March 2009. The body was designed to have oversight over mortgages and other financial instruments to protect consumers against predatory practices. She said if the agency had existed before the subprime collapse then "there would have been millions of families who got tangled in predatory mortgages who never would have gotten them." HuffPost's Ryan Grim <a href="" target="_hplink">reported</a>: <blockquote>Without all these toxic assets on banks' balance sheets, the institutions wouldn't be on the brink of collapse and the recession would be more manageable. "Consumer financial products were the front end of the destabilization of the American economic system." Sen. Charles Schumer's cosponsorship of the bill is notable because of his proximity to Wall Street. The bill's merit, the New York Democrat said, is that it regulates the actual financial product rather than the company producing it.</blockquote>

  • Geithner Opposes Her Heading CFPB

    Tim Geithner expressed opposition to her nomination for the Consumer Financial Protection Bureau, <a href="" target="_hplink">reported</a> HuffPost's Shahien Nasiripour. Geithner thought Warren's views on the big banks and Wall St. were too tough. Warren's oversight of the Treasury department as a watchdog for TARP apparently irked Geithner, agressively <a href="" target="_hplink">questioning him</a> during Congressional hearings: <blockquote>While her grilling of Geithner in September, over what members of Congress have called the "backdoor bailout" of Wall Street through AIG, inspired the "squirm" video, just last month Warren pressed Geithner on the administration's lackluster foreclosure-prevention plan, Making Home Affordable. Criticizing him for Treasury's failure to keep families in their homes, she questioned Treasury's commitment to homeowners.</blockquote>

  • Ready For A Fight

    Elizabeth Warren <a href="" target="_hplink">reiterated her desire</a> for a strong Consumer Financial Protection Agency to HuffPost's Shahien Nasiripour: <blockquote>"My first choice is a strong consumer agency," the Harvard Law professor and federal bailout watchdog said in an interview with the Huffington Post. "My second choice is no agency at all and plenty of blood and teeth left on the floor."</blockquote>

  • Named Interim Chief Of CFPB

    In September of 2010, HuffPost's Ryan Grim <a href="" target="_hplink">reported</a> that Elizabeth Warren was being considered as a candidate for interim director of the Consumer Financial Protection Bureau. Days later the announcement was <a href="" target="_hplink">official</a>. The move allowed Warren to set up the groundwork for the agency immediately without risking a GOP filibuster of her nomination, a response that seemed certain giving the <a href="" target="_hplink">public opposition expressed</a> by some Republican senators. When it came time to put forth an appointment for a longterm CFPB chief, Warren was overlooked, partially because she was seen as unfeasible, but also, HuffPost's Shahien Nasiripour <a href="" target="_hplink">reported</a>, because she was a divisive figure within the Obama administration: <blockquote>Ultimately, Warren wanted the job, allies said. And near-united opposition from Senate Republicans -- 44 of them signed a letter saying they'd oppose any nominee -- should have made it easier for Obama to nominate her, since the Republicans publicly said they wouldn't support anyone for the role. Instead, the Republicans made it easy for the White House to deflect questions about the administration's lack of support for Warren. Asked how she squared the administration's public statements with its private ones, Warren declined. "I really have to say, I'm just not there. I'm not in the intricacies of the political part of this, and I can't comment," Warren said Monday. "The truth is I don't know anything about it."</blockquote>

  • Chats With HuffPost About Bureau

    In October 2010, shortly after being tasked with building the groundwork for the CFPB, Warren stopped by HuffPost to chat with Ryan Grim and Shahien Nasiripour "This is the first real agency we've built in the 21st century -- well, there's Homeland Security, but one for the people. And it means we ought to think differently," said Warren. "The government can talk to people and people can talk to the government differently than when the Consumer Product Safety Commission was built, or when the FDA was built. And if we do this right, that should change the whole dynamic of who this agency really is." HuffPost's Ryan Grim <a href="" target="_hplink">reported</a>: <blockquote>By gathering information, contracts and documents from homeowners and consumers, and allowing watchdog groups and individual concerned citizens access to those documents, the agency can exponentially expand the manpower it has to review the operations of banks and lenders. The goal would be to become aware of a particularly fraudulent practice before it is rampant and insulates itself in the financial services industry.</blockquote> For full video of the interview, click <a href="" target="_hplink">here</a>.

  • GOP Calls Her A Liar

    In May, Warren was called to testify before a House subcommittee and defend the merits of the CFPB. Some of the questions submitted by Republican representatives appeared confused and at times aggressive, leaving Warren to correct them on some basic facts about the actual purpose of the bureau. HuffPost's Mike McCauliff <a href="" target="_hplink">relays</a> one particularly contentious moment: <blockquote>The subcommittee chairman, Rep. Patrick McHenry (R-N.C.), began the proceedings by suggesting Warren had lied to the committee in a previous hearing that had questioned the CFPB's role in offering advice to state attorneys general negotiating a settlement with abusive mortgage servicers. At the time, Warren said she was proud her agency had been able to help, at the request of the treasury secretary. But McHenry brought up the memo again, suggesting it showed that she hid a larger role in the negotiations from Congress. "This is our job, and we're trying to do our job, to be helpful to other agencies, and to help those agencies to hold those who break the law accountable," Warren said, repeating that she was proud of the work.</blockquote>

  • Announces Senate Run

    Elizabeth Warren <a href="" target="_hplink">announced</a> on September 14, 2011 that she was running for the United States Senate seat currently held by Scott Brown (R-Mass.) "After listening to people all across our state who know that we can do better, folks who are frustrated like I am that Washington just doesn't get it, I'm running for the Senate so I can fight every day for Massachusetts families," Warren <a href="" target="_hplink">wrote on The Huffington Post</a>.

  • Fundraising

    One month into her campaign to secure the U.S. Senate seat currently held by Scott Brown in Massachusetts, Elizabeth Warren raised $3.15 million, largely <a href="" target="_hplink">from small donations</a>. According to a campaign email, 96 percent of donations were under $100. "These are pretty amazing numbers for our first official finance report, raised in a very short period of time," she said in an email to supporters. Warren's campaign has also attracted <a href="" target="_hplink">large liberal donors</a>, including colleagues from Harvard and well-known liberal donors like George Soros, Barbra Streisand, and DreamWorks CEO Jeffrey Katzenberg. Warren <a href=" " target="_hplink">raised</a> an impressive $5.7 million in the fourth quarter of 2011. In early January, the candidate's <a href="" target="_hplink">money bomb</a> pulled in more than $100,000 in just one weekend.

  • Historic Agreement

    Elizabeth Warren and Sen. Scott Brown (R-Mass.) <a href="" target="_hplink">signed a pledge</a> to curb third-party attack ads. If either campaign breaks the agreement, they would donate half the cost of the outside ad to a charity of their opponent's choice. "This may not work," <a href="" target="_hplink">Warren said in an email to supporters</a>. "But there's enough at stake to make it worthwhile to try to take back this election."