The contrasting ads are characteristic of a two-fold strategy Brown seems to be launching at this point in the campaign — to maintain his favorables in the state, which remain his greatest advantage over Warren, while attempting to raise doubts in voters' minds about her character and trustworthiness.
Brown's campaign has accused Warren of using claims of her Native American heritage to advance her career and receive preferential treatment, and it asserts that there's no available independent verification that she has such a background. She has said before, and repeated during last week's debate, that she was merely repeating stories she had been told by family that she didn't think to question.
The most recent poll, out from WBUR and MassINC, gave Warren a 5-percentage-point lead over Brown, and President Obama's substantial lead over Mitt Romney in the state is likely to boost Warren on Election Day. But Brown remains well-liked statewide, while Warren is still combating the perception that she's aloof and professorial.