MANCHESTER, N.H. — Elizabeth Warren lashed out Thursday at her moderate Democratic rivals seeking the White House, accusing them of naively accepting Republican calls for unity rather than standing up to the rich while too quickly bending to the whims of their own wealthy donors.

The Massachusetts senator strongly defended her progressive vision for the nation built on using new taxes to extend benefits such as universal child care and health coverage. Warren didn’t name former Vice President Joe Biden or Mayor Pete Buttigieg of South Bend, Indiana, but she was clearly taking aim at them — suggesting that long-simmering tensions between the party’s centrist and progressives wings are boiling over.

“Unlike some candidates for the Democratic nomination, I’m not counting on Republican politicians having an epiphany and suddenly supporting the kinds of tax increases on the rich or big-business accountability they have opposed under Democratic presidents for a generation,” Warren said during a speech at the New Hampshire Institute of Politics. “Unlike some candidates for the Democratic nomination, I’m not betting my agenda on the naive hope that if Democrats adopt Republican critiques of progressive policies or make vague calls for unity that somehow the wealthy and well-connected will stand down.”

The comments were some of the most strident for a candidate who rode a steady rise in the polls throughout the summer to become a front-runner but lately has seen that support stall as Buttigieg and Biden seem to have grabbed some momentum.

“We know that one Democratic candidate walked into a room of wealthy donors this year to promise that ‘nothing would fundamentally change’ if he’s elected president,” Warren said, referring to past comments by Biden.

Referencing Buttigieg’s fundraising prowess, she said, “We know that another calls the people who raise a quarter million dollars for him his ‘National Investors Circle,’ and he offers them regular phone calls and special access.”

“When a candidate brags about how beholden he feels to a group of wealthy investors,” Warren added, “our democracy is in serious trouble.”

Warren remains bunched near the top of the polls with Biden, Buttigieg and Vermont Sen. Bernie Sanders, the race’s other strong progressive voice. But rather than contrasting herself with Sanders to shore up the Democratic left, she has increasingly gone after the mayor and the former vice president — as well as billionaire and former New York City Mayor Michael Bloomberg. The division is growing as the first votes of the Democratic primary near, and there’s mounting concern that no clear front-runner will emerge from the initial slate of contests.

In response to Warren’s speech, Buttigieg senior adviser Lis Smith suggested that the senator may ultimately undermine Democratic unity.

“Sen. Warren’s idea of how to defeat Donald Trump is to tell people who don’t support her that they are unwelcome in the fight and that those who disagree with her belong in the other party,” Smith said.

Buttigieg, appearing on CBS on Thursday, took his own shot at one piece of Warren’s policy agenda, saying, “I’m not going to promise that we can just wave away all student debt.”