Donald Trump is the undisputed frontrunner for the GOP nomination -- and on a debate stage Thursday his rivals acted like it. The barbs flew early and often, as Marco Rubio and Ted Cruz sought to rattle Trump and cast him as unreliably conservative ahead of Michigan's first-in-the-Midwest primary. John Kasich, struggling for relevance, insisted that the race would shift his way as it shifts north. Here's a look at the standout moments:

1. Rubio latches onto Trump's new squishiness.

In their first volley, Rubio attacked Trump’s conservative credentials, saying that primaries and caucuses show that "two-thirds" of Republican voters aren't comfortable with him.


"The reason why is because we are not going to turn over the conservative movement or the party of Lincoln or Reagan, for example, to someone whose positions are not conservative," the Florida senator said of Trump, the current national poll-leader. "To someone who last week defended Planned Parenthood for 30 seconds on a debate stage. To someone...who thinks the nuclear triad is a rock band from the 1980s."

Rubio and Cruz both took Trump to task for reportedly showing flexibility in his supposedly hardline immigration position, asking him to tell The New York Times to release the tapes from an off-the-record discussion. Trump refused.

2. Thanks but no thanks, Mitt.

Asked to respond to Mitt Romney's strategy to prevent Donald Trump from getting the nomination -- in part by urging Republicans to back Rubio in Florida and Kasich in Ohio -- Kasich shrugged off the former GOP nominee.

"Mitt Romney's a great guy but he doesn't determine my strategy," Kasich said.

He said the race had now entered "March Madness" and that he would perform better when he gets to "my turf."

3. Trump's clothing line

Trump and Rubio wasted little time reengaging their nasty personal digs at each other, and at one point Rubio ripped Trump for manufacturing his clothing line overseas -- and criticized Trump for accepting a loan from his father.

Trump shot back.

"This little guy has lied so much about my record," he said. After a brawl, Trump said he would start migrating his clothing line operations stateside. But he suggested that trade policies have discouraged clothing manufacturing in the United States.

Later, Wallace told Trump he had a policy question.

"Let's see if he answers it," Rubio interjected.

4. "Nice to be with you, Megyn"

It was a long-awaited meeting. Moderator Megyn Kelly, who drew Trump's fury with tough questioning in his first debate appearance in August, looked him in the eye -- and said "Hi."

"Nice to be with you Megyn," he said, after a pregnant pause.

"Good to have you here," she said, smiling.

5. Trump gets phallic

After Rubio defended his personal digs on Trump of late, Trump responded that he was most taken aback by Rubio's criticism of his hands, calling them small. Rubio, he noted, had insinuated that "If they're small, something else must be small." Trump held up his hands, asking the audience whether they really considered his hands small.

"I guarantee you there's no problem," he said.

6. "Lyin’ Ted"

The sniping wasn’t just exclusive to Trump and Rubio.

Cruz implied that Trump was “misleading the American people” about his immigration views because he refuses to release the tape of an off-the-record conversation with The New York Times in which he discussed negotiating over his hardline position.

“If you didn’t tell them that, the tapes will prove you innocent,” Cruz said.

Trump responded, “You’re the lying guy up here” and the two spoke over each other repeatedly.

“I’ve given my answer, lyin’ Ted,” Trump concluded.

7. Trump defends flip flops

Fox played a series of videos showing Trump changing his mind on crucial issues -- the decision to go to war in Afghanistan, whether the country should bring in Syrian refugees and whether George Bush lied about weapons of Mass destruction in Iraq.

"There are many other examples," Kelly said.

Trump, after defending his switches, argued that leaders should be flexible.

"I have a very strong core," he said. "I've never seen a successful person who wasn’t flexible."

8. Kelly takes on Trump

One of the fiercest exchanges of the entire night occurred between Trump and Kelly.

After addressing Trump University -- the school Trump founded that has been sued by former students as a scam -- Kelly was ready with an on-the-spot fact-check. She noted that "so-called little guys" said they had been "fleeced" by Trump.

"The class has been certified and in that case you counter-sued the lead plaintiff," she said.

Trump has defended his company by arguing that its former students have largely viewed it favorably and a positive rating from the Better Business Bureau. But again, Kelly rebutted him, noting that the BBB had given it a D- rating, and she read from an appeal court decision that compared Trump University students to "Madoff victims."

"Give me a break. Let’s see what happens in court," he said,

9. Rubio pans Trump on foreign policy

Moderators teed up a softball for Rubio: Explain why Trump is unprepared to be commander in chief. And Rubio delivered a harsh assessment of Trump’s foreign policy readiness.

“Donald has not shown a seriousness about foreign policy,” he said, adding that he hasn’t shown the intellectual curiosity or the interest in learning about these very serious issues.”

Trump rebutted with an attack on Rubio: “Believe me, he is not a leader.”

“I will prove to be a great leader,” he added.

Rubio retorted, “He was pressed on a policy issue … his reaction was just to attack somebody else with a name."

10. All candidates vow to back the GOP nominee

Despite a growing movement to isolate Trump by establishment Republicans, all four Republican nominees vowed to support the GOP presidential nominee no matter who wins the primary.

"I’ll support Donald if he’s the nominee," Rubio said, arguing that he's preferable to Hillary Clinton and Bernie Sanders.

"Yes. Because I gave my word that I would," Cruz said.

Kasich agreed that he would, but added "Sometimes he makes it a little bit hard."

Trump feigned surprise when asked if he would support the GOP nominee if it isn't him. "Even if it's not me?" he wondered aloud.

After noting he had brought many new people into the party, he said, "The answer is, yes I will."

11. Kasich touts his Reagan days

Discussing foreign policy, Kasich insisted that he's the best choice to lead the nation because he was "there when Ronald Reagan rebuilt the military."

"I was there when Ronald Reagan rebuilt the economy," he continued. "I knew Ronald Reagan."

