David Jackson

USA TODAY

The political truce between Donald Trump and Ted Cruz is over, at least as far as Trump is concerned.

Cruz is "a good debater — but very strident and a lot of people aren't going to like that," Trump told MSNBC's Morning Joe on Friday, the morning after the two candidates clashed repeatedly during a Republican debate in North Charleston, S.C.

Little more than two weeks before the Iowa caucuses open the nomination process, Trump indicated he would continue to hit Cruz over his Canadian birth and his attack on the "New York values" that Trump represents.

Six takeaways from the South Carolina Republican debate

For his part, Cruz says he still likes Trump — "I'll sing his praises" — but added that he will defend himself against attacks by Trump, Marco Rubio, or any other Republican candidate.

"I'm not going to sit there," Cruz told Fox Business Network, which sponsored Thursday's debate.

Cruz, who leads Trump in several Iowa polls, said the reason for Trump's new attacks is simple: "His poll numbers are falling."

While he was born in Canada, Cruz says he is legally eligible for the presidency because his mother was a U.S. citizen. Trump and other opponents said there is legal uncertainty, and Cruz is vulnerable to a lawsuit is he is nominated.

"He's got a problem, he's got to straighten out the problem," said Trump, who flew overnight to appear on Morning Joe as it broadcast from Des Moines.

Trump also renewed his attack on Cruz for criticizing "New York values." Noting how New York has recovered from the 9/11 attacks, Trump said that "I thought it was a terrible thing for him to say."

Cruz and aides said he was referring to the fact that New York is a very Democratic city that strongly supports liberal policies.

In recent months, Trump and Cruz made a display of refusing to attack one another, but that highly publicized "bromance" has broken up just ahead of the Iowa caucuses.

The next Republican debate: Jan. 28 in Iowa.

Only seven candidates participated in Thursday's prime-time event because of polling criteria established by the Fox Business Network.

The smaller stage may have sharpened the exchanges among candidates in the sixth debate of the Republican campaign, said Alan Schroeder, a debate scholar at Northeastern University and the author of Presidential Debates: 50 Years of High-Risk TV.

Trump probably had his best debate of the campaign, Schroeder said, while Cruz seemed ill-prepared for the questions about his birthplace and comments about New York.

"Obviously, it was a very lively debate," Schroeder said. "As you get closer to the voting, things get a little more tense."