By Joseph Ax and Amanda Becker

BOSTON (Reuters) - Democratic presidential contender Elizabeth Warren sought to re-energize her White House campaign in a New Year's Eve speech on Tuesday, warning that "democracy hangs in the balance" five weeks before nominating contests begin in early February.

In her home state of Massachusetts on the first anniversary of her campaign launch, Warren said President Donald Trump would "try to cheat his way through yet another election" if he is not removed from office after his impeachment by the Democratic-controlled U.S. House of Representatives.

"In the past 12 months, the president has become bolder with his lies and more brazen in his law-breaking," said Warren, who as a U.S. senator will vote on whether to convict Trump of improperly pressuring Ukraine for political favors. "Meanwhile, the Republicans in Congress have turned into fawning, spineless defenders of his crimes."

The race for the Democratic nomination remains fluid as the calendar turns to 2020, with 15 Democrats still in the running and a majority of voters telling pollsters that they have yet to settle on a final choice. The nominating contests in Iowa and New Hampshire in early February will be critical tests of candidates' viability.

Warren's address took place in front of a crowd of nearly 700 people at a church in downtown Boston known as a gathering place for revolutionary colonists in the 1770s.

"We are a nation that fights back," she said. "Fighting back is an act of patriotism."

Warren remains a top Democratic candidate in national opinion polls but her standing slipped in autumn after a months-long surge that briefly vaulted her to front-runner status.

She is in third place behind Joe Biden, the former vice president, and fellow U.S. Senator Bernie Sanders, according to the website RealClearPolitics's national polling average.

Warren's momentum stalled under sustained attacks from more moderate Democratic candidates like Pete Buttigieg, the outgoing mayor of South Bend, Indiana, over her support for Medicare for All, the healthcare overhaul that would eliminate private insurance in favor of a single government-run plan.

In response, Warren has revised her rhetoric on healthcare, emphasizing her intention to phase in Medicare for All over several years to preserve "choice" for Americans.

She has also sought to return to the theme of economic populism that animated the early part of her campaign.

Warren, who has sworn off high-dollar fundraisers, argued on Tuesday that other candidates who "kiss the rings" of the wealthy are beholden to rich donors and corporate interests.

"The billionaires, the corporate executives and their favorite presidential candidates have one clear goal: to convince you that everything you imagine is impossible," Warren said.

While she did not name any rivals, her remarks were likely aimed at Biden and Buttigieg, whom she has previously criticized for holding high-priced fundraisers.

In recent months, Warren has seen a slowdown in her fundraising pace. The campaign said last week it had raised just over $17 million in the fourth quarter with a few days to go, lower than the $24.6 million she raised last quarter.

(Reporting by Joseph Ax and Amanda Becker; Editing by Peter Cooney and Alistair Bell)