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."