Democrats on Capitol Hill are starting to get worried about Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE.

Once convinced that a Trump nomination would be a godsend for their party, some are now warning that a general election fight against the billionaire tycoon will be anything but a cakewalk.

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Trump's unorthodox message and populist appeal, the Democrats warn, could erode their hold on working-class support and jeopardize their chances in a year when voter disenchantment with Washington is being felt nationwide.

"I've been saying for months that we should never take Trump lightly and that I do think he has appeal, to independents and blue-collar Democrats especially," said Rep. John Larson (D-Conn.), former head of the House Democratic Caucus. "He is stoking the fears. … He comes along and says, 'I'm a deal maker, I'm about getting the deal done.' And they're so fed up of seeing nothing getting done and want to see him [act] on the issues that strike to the core of their feelings."

Larson predicted his preferred candidate, former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE, would prevail over Trump once he's forced to delve more deeply into policy issues. But it won't be without a fight.

"In a race against Hillary, on the stage, drawing a bright spotlight on him and drilling down is the way that he's going to be beat, and I think she'll beat him. But for us to take him lightly would be the worst mistake in the world," Larson said.

"It's not only taking him seriously," echoed Rep. Rául Grijalva (D-Ariz.), head of the Progressive Caucus, "it's taking him as a threat."

Those warnings carry a sharply different tone from the public statements coming from Democratic leaders and campaign strategists, who are trying to tie Trump to the rest of the Republican Party. Those voices suggest Trump's nomination would be a boon, not only to the Democrats' presidential pick, but to down-ballot candidates as well.

Vice President Biden told House lawmakers in January that having either Trump or Sen. Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE (R-Texas) atop the Republican ballot would be "a gift from the lord." And following Trump's resounding victories on Super Tuesday, Rep. Ben Ray Luján (D-N.M.), head of the House Democrats' campaign arm, delivered a similar message.

“With Donald Trump moving closer and closer to becoming the Republican nominee and Hillary Clinton demonstrating overwhelming support on the Democratic side, we have a snapshot of a general election match up that will provide a clear contrast for voters and yield big Democratic victories up and down the ticket,” Luján said.

But a growing number of rank-and-file Democrats have a warning for their party leaders.

"Be careful what you ask for," said Rep. Bennie Thompson (D-Miss.), another Clinton supporter and prominent member of the Congressional Black Caucus.

"A few months ago, nobody gave Donald Trump a chance of being a serious contender. Now he's the leader. So it's incumbent upon Democrats to do what, historically, we've been good at, and that's communicating with our voters, letting them know what we stand for, and putting those apparatuses in place to move our voters to the polls."

Liberals off of Capitol Hill are sounding the alarms as well. Writing in The Nation, John Nichols recently warned that Trump's blue-collar appeal could take him all the way to the White House. And last week, after months of largely ignoring Trump's campaign, John Oliver dedicated the full 30 minutes of his weekly HBO show to unbraiding the Republican front-runner.

“At this point, Donald Trump is America’s back mole: it may have seemed harmless a year ago, but now that it’s gotten frighteningly bigger, it is no longer wise to ignore it,” Oliver said on his "Last Week Tonight" program.

The rising concerns come as Trump moves ever closer to securing the GOP nomination.

His success is ripping wide the already deep divisions within the GOP. Some top Republicans, perhaps seeing Trump as the inevitable nominee, have recently jumped on the bandwagon with official endorsements. New Jersey Gov. Chris Christie (R) was the most prominent to endorse, arguing that Trump offers Republicans the best chance of defeating Clinton in the general election.

Other Republicans are sprinting in the opposite direction, warning that a Trump primary victory would spell doom for the party, not only in the presidential race, but across the ballot.

On Thursday, Mitt Romney, the GOP's 2012 nominee, delivered a takedown of the businessman-turned-politician, and is now pushing efforts to use a contested convention to dethrone Trump, should he not win a delegate majority.

The GOP critics fear that Trump's recent reluctance to denounce a white supremacist — on top of controversial positions on immigration, spats with female journalists and questionable business dealings — will alienate women, minorities and young voters.

“This is the very brand of anger that has led other nations into the abyss,” Romney warned in an address at the University of Utah in Salt Lake City.

Through it all, Trump has emerged unscathed, with the GOP attacks serving only to boost his popularity. A recent CNN/ORC International poll found that 49 percent of Republican voters back him nationwide.

Citing Trump's seemingly Teflon candidacy, Rep. Michelle Lujan Grisham (D-N.M.) said the Democrats are facing "a serious threat."

"There's no dearth of issues that you can attach to Trump as a candidate that are problematic. But if what folks are just saying is, 'We don't care,' then that changes your strategy," Grisham said. "Voters are so disenfranchised and so angry that they're willing to vote for anything that's so different and drastic. And my own sense is they sort-of want to send Trump to us" as a punishment for gridlock.

There's a good deal of debate about the best strategy for taking on Trump. Some have suggested that the Democratic nominee should fight fire with fire, responding to his personal attacks in a similar vein. Most Democrats, however, say their front-runner Clinton should ignore the vitriol and continue to focus on issues.

Rep. Emanuel Cleaver (D-Mo.), former head of the Black Caucus, lamented the calls for Clinton "to get down in the gutter" with Trump. If she adopts that strategy, Cleaver warned, "I don't think I'd be able to vote for her."

"And I'm going to tell her that," he added.

Larson is also advocating the high-road approach.

"Stick with the issues, stick with policy, stick with the personal anecdotes about how this intercedes in people's lives," he said. "That's the only way that you can really impact a demagogue."