(CNN) – The battle for U.S. Senate in Massachusetts, which seemed over the summer to be the nation's most civil political race, has morphed into one of the ugliest.
In just the last three days, dueling attack ads have reignited the debate over Elizabeth Warren's Native American heritage, with Sen. Scott Brown, Warren's Republican rival, suggesting in his spot that Warren is untrustworthy.
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Warren fired back, producing an ad claiming Brown was attacking her family.
On Tuesday, videotape surfaced of Brown staffers making offensive gestures at a Warren rally that seemed to mock Native Americans. The principal chief of the Cherokee Nation called the "war whoops" and "tomahawk chops" "offensive and downright racist," saying the actions "perpetuate negative stereotypes and continue to minimize and degrade all native peoples."
Asked about the tape Tuesday at a press availability at Boston's South Station, Brown said he hadn't seen the footage but did not support the actions as described by reporters.
"That's not something I condone. It's certainly something that if I am aware of it, I'll tell that member to never do it again," Brown said, before launching into an attack on Warren's claims to Native American heritage.
It's all a sudden move to the low road for two candidates who seemed determined earlier in the race to stick to the issues. Both signed a pledge in January calling for super PACs and other third party groups to stay out of their contest, hoping to keep focused on issues that affect the Bay State. The campaigns also promised not to coordinate with any groups planning to launch "sham ads" during the course of the campaign.
Brown stuck largely to a message of bipartisanship on the stump and in interviews, keenly aware of Massachusetts' moderate Republican base he hopes to win in November.
In an election year when most Republicans running for re-election boast about their votes blocking President Barack Obama's agenda, Brown has bragged about helping the Democratic incumbent.
"I can name a litany of Democratic-sponsored bills that never would have passed had it not been for me," Brown told CNN in July. "The president had called me. The vice president calls me. Secretary [of State Hillary] Clinton calls me for my vote all the time."
That claim was followed up with a fact-check in the Boston Globe, which reported Clinton and Brown had spoken twice by phone. He spoke with Biden and Obama each once by phone, but has held in-person meetings more frequently with the president and vice president.
At the beginning of September, Brown produced an ad showing him shaking hands with Obama.
"Good job," the president is seen saying in the spot.
Warren, for her part, focused largely on her record as a consumer advocate, rather than on Brown, using her speeches to tout her record as the brains behind the new Bureau of Consumer Protection.
At the Democratic National Convention, Warren delivered a strident speech that brought a packed convention arena to its feet in thunderous appreciation – but didn't mention her rival by name.
"Americans are fighters. We are tough, resourceful and creative. If we have the chance to fight on a level playing field – where everyone pays a fair share and everyone has a real shot – then no one can stop us," Warren said.
Until Warren's prime-time address at the DNC, the race remained close. But a pair of surveys after the speech showed the Democratic candidate pulling ahead of Brown, 48%-44% in a Suffolk University/7News poll and 50%-44% in a Western New England University poll.
And thus the onslaught began, with Brown launching full tilt into an attack on Warren's claims to Native American ancestry.
Last week, Brown opened an hour-long debate with the issue, claiming Warren wasn't being totally transparent about why she was listed at Native American when she was a professor at Harvard Law School.
"Professor Warren claimed that she was a Native American, a person of color and as you can see, she's not," Brown said. He repeated his call for his opponent to release records showing whether she had claimed Native American heritage on job applications.
"When you are a U.S. Senator, you have to pass a test and that's one of character and honesty and truthfulness. I believe and others believe she's failed that test," Brown said.
That attack was followed by the television spot, which Warren rebutted with a spot of her own.
"Let me be clear. I never asked for or never got any benefit because of my heritage," she says, addressing Brown's central concern. "The people who hired me have all said they didn't even know about it."
"Scott Brown can continue attacking my family, but I'm going to keep fighting for yours," the former Obama administration official concludes.
On Wednesday, Brown made another attempt at cracking Warren's image as a crusader for the middle class, addressing a letter to his rival asking her to disclose her outside legal work while she was working as a professor at Harvard Law School.
Brown cites reports that Warren provided legal counsel to Travelers Insurance and LTV Steel while she was a professor, which Brown claims in his letter "is at odds with the image you portray on the campaign trail fighting for the middle class and the little guy."
"During this campaign, you have accused others of siding with big corporations," the GOP senator concludes. "As it turns out, you are the only candidate in this race who has stood on the side of large corporate interests over the middle class."
At a press conference in Boston Wednesday, Warren countered Brown's claims by pointing out her rival has "never released the name of a single client," according to the Boston Herald.
"I think what he's trying very hard to do is not talk about his voting record. So let's keep count, so far he's come after my family, he's come after my teaching, he's come after when I've gone to the U.S. Supreme Court," she said.
CNN's Ashley Killough, Dana Bash and Alison Harding contributed to this report.