The idea is to undercut Brown’s image as an independent, go-his-own-way Republican. Top Democrats and party officials say Warren’s tougher ad campaign against Brown is aimed at voters who are likely to vote for President Barack Obama but remain unsure about the Senate race.

“I’ve never seen him give us a vote when we really needed it,” Schumer said of Brown. “He’ll always give us a vote when the votes are 62-38 or even 55-45. But he never gives us a vote.”

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Neither Schumer nor his staff would comment on the senator’s recent conversations with the Warren campaign.

Brown and his allies say Warren’s new tack is a sign of desperation.

“The people of Massachusetts deserve and expect better, especially from a first time candidate who initially claimed not to like attack ads,” Brown said in a statement.

Brown won his seat in a stunning upset in 2010, in a low-turnout special election triggered by the death of Democratic Sen. Ted Kennedy. Turnout is expected to be much higher in November with Obama and former Massachusetts Gov. Mitt Romney on the ballot — a scenario that should favor Warren in the Democratic-leaning state.

Rep. Michael Capuano (D-Mass.), who plans to attend a Boston rally with Warren on Saturday, said if Brown is reelected, he’ll simply be pushed “aside” by a conservative GOP majority.

“He’s got this moderate persona going, people accept it to some extent,” Capuano said. “He’s a good candidate, he’s had great ads as far as I’m concerned … but they don’t tell the whole story.”

A race of mudslinging and negativity isn’t what either camp professed to have in mind at the outset. In January, Brown and Warren signed a pledge to keep what certainly would have been a barrage of super PAC ads off the airwaves.

To some surprise, the accord, the only one of its kind in the country, has held.

But since the race could determine which party controls the Senate next year, it seemed only a matter of time before one of the candidates went negative.

Many observers assumed it would be Brown whose hand was forced. But in the great Bay State ad war parlor game, Warren blinked first.

“In a close race, it’s a game of chicken. There are some risks for both of them to go negative,” said Boston-based Democratic strategist Mary Anne Marsh. “The fact that the campaign to date, including the advertising, has been the tamest in the nation, just reinforces that belief.”

Marsh said Warren has undoubtedly connected with voters on the stump. But the Democratic strategist said Warren’s personal story hasn’t been conveyed thoroughly enough in her air campaign.

“The Elizabeth Warren on the stump is the one we need to see in the ads,” Marsh said. “She can connect by telling her story as an unmarried young mother working her way through school. I think by telling that story again and again in ads … that’s compelling.

“She needs to keep connecting with people personally. Because if people like you personally, they’re going to believe you,” Marsh added.

John Bresnahan contributed to this report.