The potential entrance of former New York City Mayor Michael Bloomberg into the 2020 presidential race marks perhaps the most significant sign yet of rising anxiety among prominent Democrats over the current candidate field.

Sens. Elizabeth Warren and Bernie Sanders are locked in a race to the left that could leave either struggling to attract the center in a general election, and former Vice President Joe Biden is facing lingering doubts about his campaign's strength.

The billionaire Bloomberg, despite previously ruling out a run, is expected to file paperwork this week designating himself as a candidate in Alabama’s Democratic presidential primary. In a statement, his political adviser Howard Wolfson said bluntly that Bloomberg was worried the current crop of 2020 hopefuls is not “well positioned” to beat President Trump next November.

BLOOMBERG EXPECTED TO FILE PAPERWORK TO ENTER 2020 ALABAMA DEMOCRATIC PRIMARY

"In 2018 [Bloomberg] spent more than $100 million to help elect Democrats to ensure that Congress began to hold the President accountable," Wolfson said. "And this year he helped Democrats win control of both houses of the Virginia legislature."

He added: "We now need to finish the job and ensure that Trump is defeated -- but Mike is increasingly concerned that the current field of candidates is not well positioned to do that."

That’s a view shared by a number of Democratic strategists and commentators. While Biden remains near the top of the polls, questions about his age, fitness and fundraising ability have shadowed him, and he has been savaged by the party’s left-flank for his more moderate positions on questions such as health care, immigration and even working with Republicans.

“This is a thunderclap,” Obama strategist David Axelrod tweeted about the Bloomberg news. “And not exactly a vote of confidence from leading moderate in durability of @JoeBiden campaign.”

Trump, meanwhile, predicted that Bloomberg would hurt Biden, and indicated he looked forward to the prospect of a Bloomberg candidacy.

“There’s no one I would rather run against than Little Michael,” Trump told reporters, while also casting doubt on Bloomberg's ability to prevail in the primaries.

It remains unclear whether Bloomberg, running from the center-left, could command the kind of primary support necessary to rise in the packed field, even spending his own fortune to do it.

One possibility, as Trump suggested, is that he'd draw just enough support from Biden to boost a more liberal candidate such as Warren, D-Mass., or Sanders. I-Vt. -- whose big-spending policy prescriptions have also raised alarm about general election viability.

“There’s more anxiety than ever,” Connie Schultz, a journalist who is married to Ohio Democratic Sen. Sherrod Brown, told The New York Times last month. Brown has also been eyed by some Democrats as a possible late entrant to the race.

“We’re both getting the calls. I’ve been surprised by some who’ve called me,” Schultz said.

Last Friday, Warren released her “Medicare-for-all” plan, coming with an eyewatering price tag of $52 trillion (including $20 trillion in new spending over the next decade) and a promise not to raise taxes on the middle class -- a claim met with deep skepticism from across the political spectrum. But centrists were also scathing about how the plan would play out in swing states.

“This is going to cause down-ballot damage in swing districts and states if she’s the nominee,” Colorado State Rep. Bri Buentello told Politico.

Sanders, meanwhile, on Thursday released his immigration plan -- a grab bag of far-left items including a deportation freeze, full welfare access for illegal immigrants, the dismantling of federal immigration agencies, and the promise to accept a minimum of 50,000 “climate migrants” in the first year of a Sanders administration.

While those plans have been cheered by left-wing activists, it is far from clear how well they would go down in swing states any Democrat needs to win in order to beat Trump.

While current national polling suggests most of the main 2020 candidates would beat Trump next year, there are warning signs in the polling data for Democrats -- apart from the general caveat that most polling also showed Trump losing in 2016, and he didn’t.

The swing states generally show a tighter race against Trump. A New York Times/Siena College survey in six battleground states released Friday shows that Democrats there also prefer a more moderate pick. A majority, 55 percent, said they wanted a candidate more moderate than most Democrats, and 62 percent wanted a candidate who would try and find common ground with Republicans.

“With Trump looming, there is genuine concern that the horse many have bet on may be pulling up lame and the horse who has sprinted out front may not be able to win,” Axelrod told the Times last month.

The idea that Bloomberg’s entrance into the 2020 race is a response to the deficiencies of the current field has been greeted by glee by Republicans.

“Really? Another one?" Republican National Committee Rapid Response Director Steve Guest tweeted. “The fact that Michael Bloomberg feels the need to run for president underscores the weak Democrat field and shows that Democrats know they can’t compete with @realDonaldTrump in 2020.”

There are other signs that Democrats on the sidelines are nervous about the current field. Hillary Clinton has been the subject of a series of rumors that she may throw her hat in the race -- a prospect that seemed unthinkable a year ago. The Washington Post, meanwhile, reported that former Attorney General Eric Holder is considering stepping into the race too.

The consideration of any of these politicians may reflect sheer personal ambition as much as concern about the field -- but the fact they're even flirting with a run at this late stage indicates they see an opening for that reason.

It remains to be seen whether Bloomberg will jump all the way in. The next major deadline he faces is in New Hampshire, where the deadline for candidates to put their name on the ballot is Nov. 11. Bloomberg aides tell Fox News there’s no word yet on whether the former New York City mayor will come to New Hampshire to file. But they add that they'll do "everything necessary to keep all of our options open.”

Team Bloomberg is also reaching out to top Granite State Democrats. Longtime New Hampshire Democratic Party Chairman Ray Buckley told Fox News that he’s been contacted. But others have pushed back on the idea that what Democratic voters want is even more candidates to choose from.

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Longtime New Hampshire based Democratic consultant James Demers – who’s backing Sen. Cory Booker of New Jersey this cycle – said that “anybody and everybody has a right to run if they want to but I’m hearing voters more and more saying ‘I really want this field to winnow down to six or seven so I can process who’s running and they’re not asking for the field to grow.’

“So I think it’s a high-stakes game for anyone to get in this late in the game because that’s not where the voters’ heads are,” he said.

Fox News’ Andrew O’Reilly and Peter Doocy contributed to this report.