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.

“Nobody is perfect, and nobody is pure. And there’s no question that any Democrat running for president would be a whole lot better than Donald Trump," Warren said Thursday to applause. She added later: “But voters will not trust a candidate who won’t make a single difficult decision that might cut down on the access and influence of wealthy donors. And voters will be right."

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.

Warren has centered her candidacy on proposing structural changes to remake the political and economic system. She wants a 2% tax on fortunes worth $50 million-plus and a levy three times that on anyone who has a net worth of more than $1 billion. She pledges to use that tax money to offer universal child care and free tuition at public universities while wiping out most student debt for 42 million Americans and helping finance a “Medicare for All" plan providing government-sponsored health care nationwide.

But some potential hurdles of doing so were on display earlier Thursday, when the University of Pennsylvania's Penn Wharton Budget Model — which provides nonpartisan analysis of public policy proposals — released findings showing that Warren's wealth tax will raise $2.3 trillion to $2.7 trillion over 10 years. That's as much as $1.4 trillion less than Warren’s campaign estimates.

The analysis also concluded that the new taxes would cause the economy to contract 0.9% to 2.1% by 2050 and says the new tax would reduce “private capital formation” enough to drive the U.S. economy’s average wage down 0.9% to 2.3%, even affecting households not rich enough to qualify for the tax.

Warren’s wealth tax has been among the most popular — and most scrutinized — proposals of her campaign. Although Sanders has proposed an even higher tax on top fortunes, Warren's has given her an economic populist edge and even prompted crowds at her rallies to break into chants of “2 cents!”

Responding to the analysis after her New Hampshire speech, Warren said it wasn't really based on what she wants to do, saying the authors “changed provisions and then analyzed something else."

“I understand there are people who want to throw up a lot of dust around this because they don't really have any comeback to that central question and that is, ‘Why aren't we asking the folks at the very top to pitch in a couple of cents so that we could actually invest in opportunity and everyone else?'" she said.

Still, even Warren herself acknowledged the potential difficulty of her agenda, noting in her speech: “Will I have a magic wand to enact my full agenda? Of course not. No president does. I know I will have to compromise, but that’s not where we start."

She said she'd exploit Senate procedures allowing some legislation to pass with a simple majority — noting that Republicans used that to pass the tax cuts championed by Trump. But pulling that off is difficult since the chamber rules would require near-universal Democratic unity.

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."

The mayor said he personally understands the effects of taking on debt to pay for school but “that doesn’t mean I can just say we’re going to make it all go away and it’s all free now.”

In New Hampshire, Warren noted that any Democrat who has “fought for real change" has been branded a socialist or radical.

“Here’s a news flash," she said. “If you defend a corrupt system, where corporate lobbyists write the rules to squeeze out competition and hurt economic growth and undercut workers, you’re not a capitalist. You’re just a cheater.”

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