While the full field of Republican presidential candidates resumed campaigning Friday, it was Sens. Ted Cruz and Marco Rubio who emerged from last night's Fox Business debate the apparent main challengers to front-runner Donald Trump -- though New Jersey Gov. Chris Christie's feisty exchanges may have left some thinking what was once a 17-candidate scramble, now is a four-man showdown.

Or, maybe five, as former Florida Gov. Jeb Bush, who managed to get in a few shots at Trump, picked up the endorsement Friday morning of former candidate Sen. Lindsey Graham.

Trump, though, quickly dismissed the backing, tweeting that Bush’s “chances of winning are zero,” the same as Graham’s polling.

The exchanges all marked a newly aggressive campaign heading into the final stretch before Iowa.

For Trump and Cruz in particular, the Fox Business Network debate in South Carolina marked the first where real tensions showed through. Before, the two top-polling candidates essentially refused to attack each other, preserving a détente that extended to the campaign trail.

No more.

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Trump, after the debate, acknowledged in one TV interview that their “bromance” is over.

The debate could set the tone for the final two weeks before the leadoff Iowa caucuses, with the leading candidates now searching aggressively for weak spots in each other’s records. 

On Friday, the candidates spread all over the early-voting state map.

Trump is following on his strong performance with an Iowa rally Friday morning. Cruz is charging through South Carolina, while Rubio is in New Hampshire. 

Trump also has rented out space at an Iowa theater and is giving residents free tickets to a showing of “13 Hours: The Secret Soldiers of Benghazi” Friday evening, according to The Des Moines Register. The movie depicts the 2012 terror attack on the U.S. compound in Benghazi, a topic that has haunted Hillary Clinton’s presidential bid. 

With the polls tightening in the middle in the Granite State – and at the top in Iowa – nearly a half-dozen candidates are now firing at each other on a regular basis.

Rubio sustained his attacks going into Friday, launching a new TV ad that calls accusations from Bush “desperate” – and reprising his criticism on Cruz’ consistency in an interview with Fox News.

“I like Ted, we’re friends, but he campaigns as a consistent conservative. … That is not his record; he has flipped his position on birthright citizenship, on legalization of illegal immigrants,” he told Fox News.

When Rubio launched that same attack at Cruz Thursday night, though, Cruz deftly countered. 

“I appreciate you dumping your oppo research folder on the debate stage,” Cruz said, maintaining that he opposes “amnesty” while Rubio backs citizenship for illegal immigrants.

Through most of the debate, Cruz showed off his skills as practiced debater. He employed this early on to fend off a challenge from Trump over the Canada-born senator’s eligibility to seek the presidency. He accused Trump of pushing “birther theories” because the polls are tightening.

Then, in a retort reminiscent of Reagan’s famous “youth and inexperience” quip, Cruz tried to flip the script by noting some “birther theories” say a candidate must have two parents born on U.S. soil to be eligible.

Pointing out Trump’s mother was born in Scotland, Cruz said: “On the issue of citizenship, I’m not going to use your mother’s birth against you.”

Trump said, “But I was born here … big difference.”

But Trump, though, was seen as landing a decisive retort when Cruz criticized him for “New York values.” Recalling memories from after 9/11, Trump passionately described the “horrific” clean up and the “smell of death” in the city.

“It was with us for months, the smell,” Trump said. “And everybody in the world loved New York, loved New Yorkers -- and I have to tell you, that was a very insulting statement.”

Not to be overshadowed, Rubio and Christie both broke through on several occasions.

While Rubio and Cruz tangled over their immigration records and voting consistency, Rubio also accused Christie of endorsing “many of the ideas that Barack Obama supports, whether it is Common Core or gun control or the appointment of Sonia Sotomayor or the donation he made to Planned Parenthood.”

Christie responded by invoking a moment from a previous debate.

“I stood on the stage and watched Marco … rather indignantly, look at Governor Bush and say, ‘someone told you that because we're running for the same office, that criticizing me will get you to that office,’” he said. “It appears that the same someone has been whispering in old Marco's ear too.”

Often left on the sidelines of the rhetorical battles were Ohio Gov. John Kasich and retired neurosurgeon Ben Carson.

Carson has seen his numbers steadily slide in Iowa, though Kasich is still polling strong in New Hampshire.