Massachusetts Sen. Scott Brown launched a new ad this morning that revisited old territory.

Brown's latest ad for his Senate Republican campaign is titled "Who Knows" and criticizes his opponent, Democratic nominee Elizabeth Warren, for self-identifying as Native-American to employers.

The 30-second spot features a series of clips from news reports about the controversy, which surfaced in the spring. Warren, who serves as the Leo Gottlieb Professor of Law at Harvard, came under fire after the Boston Herald reported on a 1996 article from the Harvard Crimson in which Warren was cited as Native-American.

Warren eventually conceded that she did self-identify this way to both Harvard and the University of Pennsylvania, where she previously taught, but only after she was hired.

The ad is the latest in a string of negative ads from both candidates, who after months of relative civility and positive messaging, have taken off their respective gloves. Warren, 63, was the first to go negative earlier this month with an ad titled "Fighter," in which she criticized Brown, 53, for siding with large corporations and "big money guys."

Polls show the two candidates in a close race. ABC News rates it a toss-up.

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