"Isn't that great? I'm very proud of what we did — we did — together," said Brown, emphasizing that Warren deserved praise for keeping her end of the bargain. "Can you imagine another $30 million of negative ads on the air?"
Warren declined to comment on the advertising pledge.
Massachusetts Secretary of State William Galvin on Monday projected 3.1 million of the state's roughly 4.2 million registered voters, about 73 percent, will go to the polls, matching turnout in 2008, the last presidential election year.
If turnout were to reach 3.2 million, it would mean about 1 million more votes being cast than in the January 2010 special election that Brown won.
The presidential and Senate contests seem to be driving interest in the election. Boston has registered 28,930 new voters since September's primary. Statewide since February, nearly 232,000 new voters have been added to the Massachusetts rolls.
Voters are also being asked to decide three ballot questions.
Question 1 would require automakers to share diagnostic and repair information with independent mechanics, while Question 2 would allow doctors to prescribe life-ending medication at the request of certain terminally ill patients.
Question 3 would allow marijuana to be used for some medical purposes.
There are also a number of high-profile congressional races.
Voters in the 4th Congressional District will choose between Democrat Joseph Kennedy III, making his first run for office, and Republican Sean Bielat to fill the seat being vacated by Rep. Barney Frank, a Democrat.
The fiercest congressional contest is in the 6th District, where Republican Richard Tisei is challenging Rep. John Tierney.
Tisei has accused the seven-term Democratic incumbent of being dishonest with voters about what he knew of an illegal offshore gambling ring run by two brothers-in-law.
Tierney has faulted Tisei for using what he called smear tactics and said he's sold out to tea party extremists.
If he wins, Tisei, who is gay, would be the first Massachusetts Republican elected to the House since 1994.
Voters will also be choosing state senators and state representatives, although Democrats' firm hold on power in the Massachusetts Legislature seems unlikely to be loosened, with Republican candidates running in less than half of the legislative seats.
Polls are open Tuesday from 7 a.m. to 8 p.m.
Salsberg reported from Lowell.