The Republican race is heating up, and leading candidates took their final jabs at each other ahead of Thursday night’s Fox Business Network debates – with Donald Trump calling Ted Cruz’s Canadian birthplace a “problem” and Jeb Bush challenging Trump’s fitness to lead in a tough, new ad.  

The debates Thursday night in South Carolina are among the last before the leadoff Iowa and New Hampshire contests.

The tight calendar is fueling new tensions in the race, with Trump trying to knock Cruz down a peg as the Texas senator challenges the billionaire businessman for the Iowa lead, as several candidates in the tier below them step up their attacks on virtually any rival near them in the polls.

The latest shot came from Bush, who is spending heavily from his war chest in a bid to recover from his steep slide in the polls. He released a new TV ad Thursday hammering Trump for at one point appearing to mock a reporter’s disability. It shows a clip of Trump imitating the reporter, and then cuts to footage of Bush calling Trump a “jerk” and highlighting the former Florida governor’s work on disabilities issues.

Trump, for his part, has denied mocking the reporter, but has long since moved past attacking Bush to needling Cruz.

On Wednesday at a Florida rally, Trump reprised his claims that Cruz might be ineligible because he was born in Canada – though legal analysts have said the fact Cruz's mother is a U.S. citizen means he can run.


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Trump told supporters his opponent has a “little problem” and, previewing his expectations for the debate, added they’ll “get into it tomorrow night.”

The Republican front-runner also has been engaged in an unusual battle on the sidelines with South Carolina Gov. Nikki Haley, who was tapped to give the official GOP response to President Obama’s State of the Union address Tuesday.

Haley urged Americans to ignore the “angriest voices,” and later acknowledged she was referring to Trump and others. She told Fox News while she considers Trump “a friend,” she does disagree “with the fact that he wants to start using religion as a way to divide people.” She was referring to Trump’s call for a temporary ban on Muslims entering the U.S. 

Trump, in response, called Haley “very weak on illegal immigration.”

And the candidate is making no apologies for his tone.

On Wednesday, Trump described his supporters as not so much a “silent majority” but a “noisy-as-hell majority.”

As before, Thursday’s debates will be broken into two parts. An evening debate of lower-polling candidates begins at 6 p.m. ET. Those qualifying for that debate are: Kentucky Sen. Rand Paul; former HP CEO Carly Fiorina; former Arkansas Gov. Mike Huckabee; and former Pennsylvania Sen. Rick Santorum. Paul has said he does not plan to attend.

Those who qualified for the prime time, 9 p.m. ET debate are:

Trump; Cruz; Florida Sen. Marco Rubio; retired neurosurgeon Ben Carson; New Jersey Gov. Chris Christie; Bush; and Ohio Gov. John Kasich.

The Thursday debates will be held at the North Charleston Coliseum and Performing Arts Center in North Charleston, S.C. 

Anchor/Managing Editor of Business News Neil Cavuto and Anchor/Global Markets Editor Maria Bartiromo will moderate the prime-time debate. The earlier debate will be moderated by anchors Trish Regan and Sandra Smith.