Trump and Cruz to clash in GOP debate

Front-runner Donald Trump will be standing at centre stage at Tuesday night's Republican presidential debate, but a rising Ted Cruz will be in the spotlight.

With just six weeks until the primary contests begin, Trump still sits atop the field of candidates vying for the 2016 Republican nomination, 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 US.

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.

The state-by-state primary contests begin in February and will last for months before each party formally nominates their candidate over the summer. The presidential election is in November.

The Rising Republican

Cruz is challenging Trump's lead in Iowa, the Midwestern state that kicks off the primaries.

A champion of the conservative tea party movement, the Texas senator has the money and campaign infrastructure to compete deep into the Republican primary season.

Those assets now make him a target for his rivals, most notably Trump and Florida Senator 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 criticised 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."