Texas Sen. Ted Cruz enters Tuesday's Republican presidential debate with a burst of momentum, a robust campaign infrastructure and a target on his back.

It's a shift for Cruz, whose campaign got off to a low-key start. Now, he's challenging Donald Trump for the lead in the kickoff Iowa caucus and casting himself as a more electable alternative who still shares the bombastic billionaire's anti-Washington appeal.

Trump and Cruz have been friendly on the campaign trail for months. But signs of a split have emerged in recent days, with Cruz appearing to question Trump's judgment at a private fundraiser, according to audio obtained by the New York Times, and Trump calling Cruz "a little bit of a maniac."

"Looks like @tedcruz is getting ready to attack," Trump wrote on Twitter last week. "I am leading by so much he must. I hope so, he will fall like all others. Will be easy!"

Trump didn't go after Cruz by name during a Las Vegas rally on the eve of the debate, but said the primetime faceoff could turn messy.

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"I am giving them a chance for them to making total fools of themselves in front of millions of people," Trump said, adding that he's expecting to be attacked. "This will not be like an evening in paradise for me. Do we agree?" he asked.

Another intriguing dynamic in Tuesday's prime-time debate in Las Vegas involves Cruz and Florida Sen. Marco Rubio, another first-term senator and Cuban-American.

Cruz and Rubio have been sparring from afar for weeks, particularly over national security, which is now a top campaign issue following the attacks in Paris and San Bernardino, California. Rubio has tried to brand Cruz as an isolationist and has criticized his support for ending the bulk collection of Americans' phone records, saying it weakens the government's ability to identify potential terrorists.

"There are some differences in policy," Rubio said of Cruz in an interview Monday with the Associated Press. "I think we need to be the national security party, the party of strong national security, committed to ensuring we have the strongest military force in the world."

More broadly, Rubio's campaign is eager to cast Cruz, who prides himself on being a conservative "truth-teller," as a politically expedient flip-flopper who is willing to say whatever is necessary to win an election. Rubio's campaign has raised questions about the consistency of Cruz's positions on sending troops to the Middle East, as well as what they consider ambiguous answers he gives on whether he would allow immigrants living in the U.S. illegally to stay here.

Cruz's campaign has dismissed Rubio's criticism, with spokesman Rick Tyler saying, "Nobody believes Senator Cruz is weak on national defense and security."

Tuesday's debate will be the first for Republicans in more than a month, and also the first since the attacks in France and California. The four previous GOP debates had record viewership, giving the candidates an unparalleled platform to reach voters.

With just six weeks until voting begins, the broad contours of the race remain consistent. Trump still sits atop the GOP field, displaying a talent for connecting with voters frustrated with Washington and on edge about the threat of terrorism. He continues to make controversial comments that might disqualify more traditional candidates, including his recent call for a ban on Muslims entering the U.S.

More experienced politicians are still struggling to break through in the crowded field. Some party leaders fear that unless the GOP establishment wing quickly rallies around one candidate, the nomination could go to Trump or Cruz — candidates they see as all but unelectable in a contest with Democratic front-runner Hillary Clinton.

Still, there have been some signs of movement in early voting states since Republicans last took the debate stage.

Cruz's rise in Iowa has been accompanied by a precipitous decline for Ben Carson, the retired neurosurgeon who has struggled with inexperience on national security matters. New Jersey Gov. Chris Christie is enjoying increasing support in New Hampshire, which helped him regain his spot on the main debate stage after being downgraded in November.

"I've been making executive decisions for 13 years and been held accountable for them," Christie said in an interview with The Associated Press. "There's no other way to get ready for the presidency than that; you have to have executive experience to be successful."

Also on the main stage Tuesday night will be Florida Gov. Jeb Bush, former business executive Carly Fiorina, Ohio Gov. John Kasich and Kentucky Sen. Rand Paul. Four lower-polling candidates will appear at an earlier event: former Arkansas Gov. Mike Huckabee, former Pennsylvania Sen. Rick Santorum, former New York Gov. George Pataki and South Carolina Sen. Lindsey Graham.

For struggling candidates, the debates will be one of their last opportunities to change the trajectory of their campaigns before voting begins in Iowa Feb. 1. The stakes are particularly high for Bush, who was once seen as a front-runner for the Republican nomination and has raised more money than any other candidate.

Bush has scoffed at suggestions that he should drop out of the race before voting begins. But that idea is likely to build if he delivers another lackluster debate performance.

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