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Three big takeaways from the raucous Republican debate in South Carolina

Thursday’s Republican debate showed what it looks like when candidates stop being polite, and start getting real.

NORTH CHARLESTON, South Carolina — No more hesitation, no more alliances, no more happy talk about how proud every candidate is to be on stage with such great colleagues. Thursday’s Republican debate was what it looks like when candidates stop being polite, and start getting real.

The more than two hour live event, which was marked by regular conflict between the front-runners, gave a strong idea of what the closing phase of the campaign looks like heading into the final stretch before voting begins in Iowa on Feb. 1. Here are three of the biggest themes to watch going forward.

The Trump/Cruz nonaggression pact is over

Just one debate ago, Donald Trump and Texas Sen. Ted Cruz were practically renewing their vows onstage after a minor tiff in the press. That awkward marriage finally collapsed Thursday as Trump’s ongoing effort to sow doubts about Cruz’s eligibility for the presidency finally pushed Cruz to respond.

Cruz delivered a strong answer out of the gate when he pointed out that Trump himself had said earlier in the year that he considered the legal issue over Cruz's Canadian birthplace settled. “Since September the Constitution hasn’t changed — but the poll numbers have,” Cruz said.

RELATED: Cruz brings the fight to Trump on presidential eligibility

But Trump pushed back hard, ignoring boos and murmurs from the audience, making the case that his only concern was that the question would prompt Democratic lawsuits: “I choose him as my vice presidential candidate and the Democrats sue because we can’t take him along for the ride,” Trump said. “I don’t like that.”

They also had an exchange over Cruz’s accusation that Trump embodies “New York values,” which prompted moderator and New Yorker Maria Bartiromo to ask him just what he meant. Cruz said South Carolinians understood New Yorkers “are socially liberal, are pro-abortion are pro gay-marriage, focus around money and the media.”

That prompted an unusually emotional response from Trump, who recounted the horrific 9/11 attacks and the “smell of death” that lingered in downtown for months afterwards as fires still raged at Ground Zero and how New Yorkers cleaned up the damage and rebuilt. “I saw something that no place on earth could have handled more beautifully, more humanely than New York.” Cruz, after bashing New Yorkers moments earlier, awkwardly applauded as Trump spoke.

Everyone is Donald Trump now

Whether or not he wins, Trump has defined the Republican primary and the debate showed just how far he’s shifted the conversation. His rivals — even the supposedly more moderate candidates running on their appeal outside the party — are adopting a darker tone, more bellicose rhetoric, and shifting their positions to the right as the contest continues.

RELATED: Trump claims even some Muslims support his ban

Sen. Marco Rubio hinted at a possible shift from his longtime support for a robust legal immigration system. Asked to defend his calls to issue more green cards on economic grounds, Rubio instead pivoted to national security and said that “the entire system of legal immigration must now be re-examined with an eye to security” out of fears of ISIS infiltration. In an earlier segment, candidates tripped over each other to take a tougher line against incoming refugees.

During one answer on guns, New Jersey Gov. Chris Christie called President Obama “a petulant child” and said that the American people are “going to kick your rear end out of the White House come this fall.” Later he railed against “sanctuary cities” for undocumented immigrants.

One wonders if any of these lines would have made it into the debates if Trump had decided to just stay at "The Apprentice" last year.

It’s all out war between Rubio and Cruz

If you feel like the Republican debates so far have been dominated by  insults and minor side issues, pay attention to the conversations between Rubio and Cruz. On national security, immigration, and taxes, they’ve had a series of meatier arguments that reveal genuine ideological splits within the party.

One such exchange was on taxes Thursday, where Rubio attacked Cruz’s plan for a flat tax and its imposition of a value-added tax on American goods and workers instead of a corporate tax. Cruz argued his system was more simple and efficient, Rubio argued it imposed a regressive tax on wages and prices that would be more susceptible to further hikes. 

RELATED: The Rubio-Cruz immigration war heats up

But the most significant moment was a fiery attack from Rubio in which he tried to puncture Cruz’s top asset: His perceived purity with the base. With machine-gun speed, Rubio spit out a hail of alleged areas where Cruz had changed his position in response to political pressure, with immigration high on the list, but also trade and crop insurance.

“That is not consistent conservatism, that is political calculation,” Rubio said.

Cruz icily congratulated Rubio on “dumping your oppo research folder,” and pivoted to attacking his rival again on his past support for immigration reform: “Marco supports legalization and citizenship for 12 million illegals. I opposed and oppose legalization and citizenship,” he said.

The two senators have been at each other’s throats for months and clearly see each other as rivals for an overlapping pool of conservative voters. Expect to see a whole lot more arguments between them, especially if Rubio emerges as the clear establishment favorite out of New Hampshire. That’s no sure thing, but relatively quiet performances by Jeb Bush and John Kasich sure didn’t hurt his chances.