WASHINGTON (AP) -- Front-runner Donald Trump will be standing at center stage, but candidate Ted Cruz will be in the spotlight at Tuesday night's Republican presidential debate.
The Texas senator is challenging Trump's lead in the kickoff Iowa caucuses, and he has the money, campaign infrastructure and conservative appeal to compete deep into the Republican primary season. Those assets now make him a target for his rivals, most notably Trump and Florida Sen. Marco Rubio.
Trump and Cruz have been getting along for months, and that has protected the senator from the harsh criticism the businessman has flung against other opponents. 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!"
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."
With just six weeks until voting begins, Trump still sits atop the Republican 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 party's 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.
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.
Tuesday's debate will be the first for Republicans in more than a month, and also the first since the attacks in Paris and California. The four previous Republican debates had record viewership, giving the candidates an unparalleled platform to reach voters.
AP writer Scott Bauer in Madison, Wisconsin, and Kathleen Ronayne in Wolfeboro, New Hampshire, contributed.
Follow Julie Pace at http://twitter.com/jpaceDC