WASHINGTON — They've dubbed Donald Trump a con artist, an embarrassment, a phony and a fraud. They've called into question Trump's business acumen and have publicized Trump's past legal woes.

As Trump continues his march to the Republican presidential nomination, his own party has gone all-out in a frantic quest to halt his bid. On Thursday, the GOP's last nominee, Mitt Romney, unleashed a blistering attack on Trump's candidacy, saying Trump poses a threat to the future of America's democracy.

"Here's what I know: Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University," Romney said in a speech in Utah. "He's playing the American public for suckers: He gets a free ride to the White House, and all we get is a lousy hat."

Sen. John McCain, the GOP’s 2008 presidential nominee, said Thursday he shares Romney’s concerns about Trump.

.@SenJohnMcCain echoes fellow former GOP nominee @MittRomney's concerns about Trump run: pic.twitter.com/rtl6IHuoOC — ABC News Politics (@ABCPolitics) March 3, 2016

But by attacking Trump, the Republican establishment has handed Democrats a treasure trove of video and soundbites to use in attack ads come November, should Trump capture enough delegates to become the party's nominee, making a post-primary unification between the candidate and the establishment hard to envision.

Republicans "spend six months attacking each other, calling each other liars, saying 'you’re not fit for office,'" Ben Howe, a contributing editor for the conservative news outlet RedState, told Mashable. "Of course, [Democrats] are going to use that."

If Trump had said 4 years ago the things he says today about the KKK, Muslims, Mexicans, disabled, I would NOT have accepted his endorsement — Mitt Romney (@MittRomney) March 3, 2016

Earlier in the month, Karl Rove, long-time GOP strategist and former Deputy Chief of Staff to President George W. Bush, wrote an Op-Ed in the Wall Street Journal, arguing the case against Trump. “There is still time for the non-Trump GOP majority to coalesce around a single candidate, but not much,” he warned.

On Wednesday, Sen. Ben Sasse, a freshman from Nebraska, threatened not to support Trump if he were nominee. If Trump is nominated, "I'm out," Sasse said on MSNBC's Morning Joe.

Mr. Trump’s relentless focus is on dividing Americans & on tearing down rather than building back up this glorious nation.

We can do better — Ben Sasse (@BenSasse) February 29, 2016

Some Republicans say the civil war against Trump is worth it, even if Trump ends up as their party's nominee.

"Frankly, I don’t know any sensible conservative in the country who would be rooting for Donald Trump to be elected president, if he’s the nominee," said California Republican strategist Jason Roe, who backs Sen. Marco Rubio's campaign. "So I think it is a poison worth taking because if Trump is the nominee, I think he will permanently damage the Republican Party."

So on the closing night of @TheDemocrats convention, will they re-play this #MittRomney video or let @HillaryClinton speak? @brandondarby — Jay Leeson (@jayleeson) March 3, 2016

Some Republicans, like Sasse, have even joined the #NeverTrump movement, vowing never to vote for Trump, even if it means Hillary Clinton would once again inhabit the White House. It's a stunning admission for a party that has spent decades trying to oust the Clintons from power.

"From my perspective, the only differences between Hillary Clinton and Donald Trump is she has a more irritating laugh, dresses a bit less well, and he's less keen on Mexicans and has been endorsed by David Duke," Liz Mair, who is part of the anti-Trump super PAC Make America Awesome, told Mashable in an email. "My principles matter to me more than any political designation ever will. If the Republican Party mutates and disfigures itself into a stealth progressive party with a white nationalist streak, it's irrelevant whether new people join it; I and millions of others will not support it if that comes to pass."

Other Republicans said a Trump nomination would be so disastrous that the comments being made by fellow Republicans is the least of their worries.

Rick Tyler, a former spokesman for Sen. Ted Cruz's campaign, said a Trump nomination would dry up donations to the Republican National Committee, rendering it unable to fund the voter mobilization operation it's been building since President Barack Obama was re-elected in 2012.

"The RNC, which is establishment controlled, would utterly collapse if Trump is the nominee, and the reason is all those donors who support the RNC would never give to a Trump RNC," Tyler said. "And Donald Trump — who we don’t know anything about how much money he actually has because he wont allow us to look at his tax returns — doesn't have the money to run a billion-dollar general election campaign, which is what running the campaign will require."

Hannity to CPAC crowd: "I am worried about what I heard from Mitt Romney today." — daveweigel (@daveweigel) March 4, 2016

Democrats, in the meantime, are watching in gleeful disbelief from the sidelines.

"I don’t know if they can put the genie back in the bottle right now," said Doug Thornell, a national Democratic strategist. "They had years to deal with their Trump problem when he was out there saying really offensive things about the president and they let him go. It was like one of those disaster films where there were all these clues that something bad was going to happen but no one takes the signs seriously until there’s a disaster, and then they're screwed."

Justin Barasky, a spokesman for Priorities USA, the super PAC backing Hillary Clinton's bid, added that, even without the Republican attacks on Trump, they were ready to take him on. The comments from Trump's own party are merely an added bonus.

"It’s a mess on their side, they made their bed and now they're sleeping in it and I don’t know what they're going to do," Barasky said. "I’d much rather be us then them."

Cameron Joseph contributed to this report