Republican debate takeaways: The Trump-Cruz feud is here to stay, and Rubio tries anger on for size

GOP Presidential Candidates Debate In Charleston

Republican presidential candidates Donald Trump and Sen. Ted Cruz of Texas participate in the presidential debate Thursday in North Charleston, S.C.

(Scott Olson / Getty Images)

Just seven Republicans appeared on the main debate stage Thursday, the smallest group yet, but the exclusivity did little to sort out the crowded field.

Here's what was notable:

The Trump-Cruz feud isn't going anywhere

The contest has settled into two distinct contests of Donald Trump vs. Ted Cruz, vs. everyone else.

The fraying bromance between Trump and Cruz spilled into the open over not just the billionaire's suggestion that Cruz may not qualify for the presidency because he was born in Canada, but also over Cruz's accusation that Trump represents "New York values."

Trump had refrained from attacking Cruz until recently, even saying last fall that the issue of Cruz's eligibility was settled. But that was before Cruz took the lead in Iowa.

"The Constitution hasn’t changed, but the poll numbers have," Cruz snapped.

"Now he's doing better," Trump acknowledged in a moment of honesty. "I didn't care before."

Only later, when Cruz turned on Trump over his "New York" sensibility, did Trump back the senator into a corner by invoking the heroism of the city in the aftermath of the Sept. 11 attacks, ending the feuding.

Rubio tries to be angrier

Rubio's greatest asset has been his optimistic message, rooted in an inspirational family story as the son of Cuban immigrants who rose to become a senator.

But as Rubio's candidacy has, like everyone else's, been stuck in Trump's shadow, his tone has sharpened -- against New Jersey Gov. Chris Christie, for example, whose record he attacked as "liberal."

"Gov. Christie has endorsed many of the ideas that Barack Obama supports," Rubio said. "... Our next president, and our Republican nominee, cannot be someone who supports those positions."

Rubio and Christie, along with lower-rung candidates Jeb Bush and John Kasich, are fighting to be the establishment-backed conservative alternative to Trump and Cruz.

Each needs a strong showing in Iowa, New Hampshire or another early state to keep his campaign afloat.

Bush emerges, a little

Debates have never much helped Bush's faltering campaign. His slump-shouldered style does not prosper in the rapid-fire format.

But Bush scored character points Thursday, if not political ones, when he attacked Trump's proposed ban on Muslims entering the U.S.

"All Muslims, seriously?" Bush asked.

Bush made the case that many of the country's Middle East allies are majority Muslim and are run by Muslims that U.S. must work with.

Trump, though, quickly distilled his argument, as he often does, to a crowd-pleasing sound bite.

"I want security for this country," Trump said.

And Bush did well mostly because he was the target of few attacks; his inability to defend against sharp attacks from other candidates has hurt him in the past.

Kasich, Carson are afterthoughts

The seven candidates qualified for the main stage because of their poll numbers, but Ben Carson and Ohio Gov. John Kasich got little attention.

"I was going to ask you to wake me up when the time came," said Carson, the retired neurosurgeon, poking fun at his own soft-spoken nature.

Kasich, meanwhile, found himself still stuck introducing himself to voters. He drew on his background, the son of blue-collar parents who has worked in Congress and the Ohio statehouse.

"You can't do on-the-job training" as president, he said.

Hillary Clinton's starring role

The evening quickly became a Hillary Clinton pile-on.

"Hillary Clinton would be a national security disaster," Bush said.

Rubio suggested she was "unfit" for the presidency because of her role as secretary of State.

"She wouldn't just be a disaster," he said. "Hillary Clinton is disqualified."

As the nominating contests draw near, the candidates are trying to sell themselves to voters as best-positioned to be beat Clinton in the general election.

Christie called a Clinton presidency "a third term" of Obama's. "She won't get within 10 miles of the White House" if Christie were president, he promised.

For more campaign coverage, follow @lisamascaro