Sen. Scott Brown (R-Mass.) is taking his questions about Democratic challenger Elizabeth Warren's Native American heritage to the airwaves in a new ad released Monday.

The "Who Knows?" ad does not feature Brown himself but runs through a series of news clips reporting that Warren listed herself as a minority while teaching at Harvard University and the University of Pennsylvania. The incumbent senator has made the issue a major part of his campaign messaging, arguing that Warren lacks credibility and should release her employment records.

Brown also raised the subject during the candidates' first Senate debate Thursday in response to a question on whether there was anything in his challenger's character that he felt was disqualifying.

"Professor Warren claimed that she's a Native American and a person of color, and she's not," Brown said. "But the fact is she checked a box."

Warren defended herself by stating that she didn't receive any professional benefit based on her heritage. She also noted that many of the faculty members who hired her have said they were not aware of her ancestry at the time.

"I think that Senator Brown is a nice guy, and this race is about the issues," Warren said. "I never asked anyone about any documentation."

But Brown pressed on, calling Warren an "excellent woman" before demanding that she "release the personnel records."

"This is about family," Warren fired back. "I can't -- and won't -- change who I am."

Warren has previously stated she believes she has Native American ancestry and, according to a Boston Globe report, a genealogist found evidence that Warren is 1/32 Cherokee.

Stephen Burbank, a professor and former dean at Penn Law School who played a role in Warren's recruitment to the faculty, issued a statement earlier this year in support of Warren. "Her appointment was based on the excellence of her scholarship and teaching," Burbank said. "I do not know whether members of the faculty were even aware of her ancestry, but I am confident that it played no role whatsoever in her appointment."

CORRECTION: An earlier version of this story said that Elizabeth Warren pointed to evidence that she is 1/32 Cherokee. In fact, the Boston Globe reported that Warren has "not documented" her Native American ancestry, but a genealogist found evidence that she is 1/32 Cherokee.

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