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."
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