Warren took the Democratic field by storm when she entered the race in September; Three of the other primary candidates dropped out within six weeks, citing the momentum behind Warren. More than one thousand supporters showed up Sunday for her first volunteer meeting in Boston.

For Warren, there is little downside to stepping up the air war early. Six weeks into her candidacy, Warren had raised more than $3 million — more than some presidential candidates — and liberal groups continue aggressively raising funds on her behalf.

"With a full year to go until Election Day, it is revealing that Professor Warren feels compelled to air hundreds of thousands of dollars in ads to try and repair the damage," said Nate Little, the state GOP's executive director. "Unfortunately for Professor Warren, voters have gotten a candid glimpse of what she really stands for that no amount of gauzy, poll-tested ads will cover up."

And Warren hasn't been the only recipient of televised attacks from outside groups. The League of Conservation Voters is spending almost $2 million to undercut Brown's record on the environment, and the League of Women Voters spent heavily earlier in the year to call out Brown for voting to reign in the Clean Air Act.