DETROIT — The Republican establishment and its last best hopes to defeat Donald Trump spent a long and extraordinary day denouncing the billionaire businessman as dangerous, a “phony” and a “con man” unfit for office. Then, when it was all over, Marco Rubio, Ted Cruz and John Kasich swallowed hard and said they’d support Trump if he wins the nomination.

Their pained commitments, at the end of the 11th GOP debate Thursday, put into sharp focus the dilemma the Republican Party faces as Trump continues to dominate the race for delegates and the campaign conversation.

Beyond harsh words, GOP leaders see little opportunity to stop Trump’s march toward the presidential nomination. Panicked party leaders are poring over complicated delegate math, outlining hazy scenarios for a contested national convention and even flirting with the idea of a third-party effort to derail Trump’s insurgent candidacy.

A tumultuous day opened with Mitt Romney, the party’s 2012 presidential nominee, making a rare public appearance to denounce Trump as a fraud who is “playing the American public for suckers.” Romney confidant Ron Kaufman, a senior member of the Republican National Committee, openly embraced the possibility of a contested convention, saying, “If that’s the only way to stop Trump, it makes sense.”

Also publicly criticizing Trump: House Speaker Paul Ryan, who was Romney’s running mate, and 2008 GOP nominee John McCain. Ryan said Trump’s more controversial ideas “disfigured” conservatism, and McCain voiced concern about Trump’s “uninformed and indeed dangerous statements” on national security.

Trump lashed back. In the debate in Detroit, he called Romney “a failed candidate” who lost to Barack Obama four years ago.

“Obviously, he wants to be relevant,” Trump said.

It was telling evidence of the party’s conundrum when Rubio and Cruz, clearly lacking enthusiasm, signaled their willingness to vote for Trump after spending much of the debate leveling withering criticism at his “flexible” policy positions and personal ethics.

The two senators, who in earlier debates had devoted considerable time to throwing sharp elbows at one another, pressed Trump aggressively on his conservative credentials, his business practices and changing policies.

Questioned a number of issues, Trump signaled a willingness to deal.

He said it was fine Florida Sen. Rubio had negotiated with other lawmakers on immigration policy.

He said he had changed his own mind to support admitting more highly-skilled workers from overseas, adding matter-of-factly: “I’m changing. I’m changing. We need highly skilled people in this country.”

He was also unrepentant about providing campaign contributions to leading Democrats, including 10 checks to Hillary Clinton.

“I’ve supported Democrats and I’ve supported Republicans, and as a businessman I owed that to my company, to my family, to my workers, to everybody to get along,” he said.

When Fox News moderator Megyn Kelly told Trump his shifts caused some people to question his core, Trump insisted: “I have a very strong core. But I’ve never seen a successful person who wasn’t flexible, who didn’t have a certain degree of flexibility.”

The bad blood among the candidates flowed freely, a steady stream of insults interspersed with the policy debate. And early on in the evening, Trump nonchalantly tossed out a crude sexual reference.

Rubio justified his attacks by saying the billionaire businessman had “basically mocked everybody” over the past year. Trump countered with a feint, saying he’d called Rubio a “lightweight” in the past but “he’s really not that much of a lightweight.”

Rubio also renewed his criticisms of Trump University, which charged students $1,495 each for seminars that would teach them the billionaire’s secrets to making it big. A lawsuit filed by the New York attorney general claims the classes fell so short of promises that it amounted to fraud.

“He’s trying to con people into giving him their vote just as he conned those people,” Rubio said.

Cruz added, “If we nominate Donald, we’re going to spend the fall and the summer with the Republican nominee facing a fraud trial.”

Trump shrugged it off, saying: “It’s a minor civil case. Give me a break.”

And he turned the tables, saying Rubio is “the real con artist.” He said the first-term senator “scammed the people of Florida” by skipping so many Senate votes.