Democratic presidential hopefuls (L-R) Mayor of South Bend, Indiana, Pete Buttigieg, Massachusetts Senator Elizabeth Warren and Former Vice President Joe Biden participate in the fifth Democratic primary debate of the 2020 presidential campaign season co-hosted by MSNBC and The Washington Post at Tyler Perry Studios in Atlanta, Georgia on November 20, 2019.

Sen. Elizabeth Warren is doubling down on her critiques of rivals Mayor Pete Buttigieg and former Vice President Joe Biden as she seeks to regain momentum in the Democratic presidential primary after weeks of faltering in national and early state surveys.

The Massachusetts progressive, who has largely avoided going after her fellow contenders since entering the race nearly a year ago, cast that strategy aside in recent weeks as she sought to establish contrasts between herself and the race's top moderates.

Now Warren plans to paint those differences in an even harsher light, using a Thursday address in New Hampshire on economic policy to blast Buttigieg for his reliance on wealthy donors, and to paint Biden as naive about the roadblocks Republicans are likely to put up in Congress should a Democrat take the White House in 2020.

"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 will say, according to excerpts of her address released by the campaign in advance.

"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," she will say.

The speech is expected to also include references to Michael Bloomberg, the billionaire former mayor of New York, who recently upended the Democratic race by spending more than $100 million of his vast fortune on a national television ad campaign.

Bloomberg's unprecedented spending spree has catapulted the mogul to fifth place in national polling less than a month after he formally entered the race.

Warren, whose decline in the polls coincided with heightened attacks on the wealthy — she started selling a branded "Billionaire Tears" mug — is expected to strike a somewhat conciliatory tone to the new entrant while using him to make the case for her wealth tax proposal.

"It's no secret that I'm not a fan of Michael Bloomberg trying to buy the Democratic presidential nomination. But there's no denying he built a successful company," she will say.

The address, at the New Hampshire Institute of Politics, comes ahead of a seven-day tour of Iowa, the first contest state, kicking off on Saturday. Warren held first place in Iowa polls for a period last month, but has since sunk to fourth place, behind Buttigieg, Biden and Sen. Bernie Sanders, I-Vt, according to an average of state surveys compiled by Real Clear Politics.

Iowa, which hosts its caucus on Feb. 3, has outsized significance in the race and is an essential part of Warren's path to victory despite its relatively small size.

The thrust of Warren's criticism of Buttigieg and Biden is that the two men do not understand the power of the forces they are up against, both in the form of wealthy entrenched interests and the Republican Party.

It comes days after a showdown between Buttigieg and Warren over transparency that resulted in Warren disclosing her past compensation for corporate legal work and Buttigieg opening up his campaign fundraisers to the press, as well as disclosing his campaign bundlers and the clients for whom he worked while a management consultant at McKinsey & Co.

Sanders and Warren do not use campaign bundlers to raise money for their campaigns. Biden, who has long had his fundraisers open to the press, has not disclosed who his campaign bundlers are, and recently dropped his opposition to a super PAC supporting his bid after posting lackluster fundraising numbers.

Despite Buttigieg's transparency push, Warren will blast the 37-year-old Democrat for taking so long to get there. She also plans to go after his "national investors circle," the name the Buttigieg campaign has given to some of its top contributors, in harsher terms than she has in the past.

"When a candidate brags about how beholden he feels to a group of wealthy investors, our democracy is in serious trouble," she will say.

Buttigieg has received more than 2 million donations, according to the campaign, with an average contribution in the most recent quarter of $32. The campaign has said his policy proposals would represent the most progressive achievements in decades if enacted.

Warren is expected to accuse Buttigieg of adopting GOP talking points in his opposition to progressive policies. Buttigieg has been criticized by some on the party's left flank, like Rep. Alexandria Ocasio-Cortez, for using similar lines of argument against proposals like free public college and "Medicare for All" as conservatives tend to, such as suggesting that Warren's and Sanders' plans would make college "free even for the kids of millionaires."

Warren will also target Biden over past comments he made at one of his own fundraisers, in which he seemed to promise the wealthy supporters in attendance that the effect of his policies on their lives would be minimal.

"We can disagree in the margins but the truth of the matter is it's all within our wheelhouse and nobody has to be punished," Biden said at the New York fundraiser in June. "No one's standard of living will change, nothing would fundamentally change."

Warren is also planning to challenge Biden's past comments suggesting that once Trump leaves office, Republicans in Congress will be likely to work with a Democratic president.

In an interview with CNBC's John Harwood earlier this month, Biden said that he believed Trump, by setting a bad example, would inspire Republicans to cooperate with him if he is elected on legislation that they opposed even while he was vice president under Barack Obama.

"There's two ways people get inspired, John. They get inspired by inspirational people like John Kennedy and they get inspired by very bad people, bad presidents like Donald Trump," Biden said.

Warren's speech could also implicitly contrast her record with Sanders, the only self-described democratic socialist in the Senate, by highlighting her record as one of the nation's leading bankruptcy scholars and the architect of the Consumer Financial Protection Bureau. Warren, who was a registered Republican until 1996, will emphasize her belief that businesses can be a positive force as long as the government imposes sufficient regulations.

"I never nursed dreams of running for office. Instead, I spent decades studying why families go broke. I became one of the nation's experts on bankruptcy. So nobody should be surprised that I believe in markets. I'm proud of what American businesses can create and I want them to thrive," she will say.

The comments seemed to strike a chord with some of Warren's rivals. An advisor to one of those campaigns suggested that the speech was a poor response to the senator's falling poll numbers.

"It turns out that a failing campaign was the one thing Elizabeth Warren didn't have a plan for," the advisor said.

— CNBC's Thomas Franck contributed to this report.