Flaring tensions between Trump, Cruz may lead to contentious Thursday night debate
Republican U.S. presidential candidates businessman Donald Trump (L) and Senator Ted Cruz (R) pose together before the start of the Republican presidential debate in Las Vegas, Nevada December 15, 2015. REUTERS/David Becker

Flaring tempers and sharp elbows may turn a debate among seven Republican presidential candidates into a seven-car pileup on Thursday with tensions between Donald Trump and rival Ted Cruz leading the way.

The sixth Republican debate, at the North Charleston Coliseum in the swing state of South Carolina, takes place at a tense time for the Republican field with the clock ticking toward Feb. 1 in Iowa, the first contest in the race to choose the party's nominee for the Nov. 8 general election.

The 9 p.m. EST (0200 GMT Friday) debate features the top seven candidates ranked by Republican voters: New York real estate businessman and reality TV star Trump, Texas Senator Cruz, Florida Senator Marco Rubio, retired neurosurgeon Ben Carson, former Florida Governor Jeb Bush, New Jersey Governor Chris Christie and Ohio Governor John Kasich.

Nearly every candidate has had a bone to pick with one or more of the others this week. Beyond the Trump-Cruz theatrics, Bush has blasted Trump and Rubio, Rubio has slammed Bush as well as Cruz and Christie, and Christie has attacked most everyone else.

"Given all the attacks that are taking place and the counterattacks, I think it will be a more lively debate than we’ve seen at this point," said Eric Fehrnstrom, who was a top adviser to 2012 Republican nominee Mitt Romney. "Donald Trump is still in complete command and the race is still about who will become his main challenger."

In an earlier debate of low-polling candidates, Republicans Carly Fiorina, Mike Huckabee and Rick Santorum largely focused their criticism on Democrats rather than each other.

They were all in agreement that President Barack Obama’s strategy toward Iran, the fight against Islamic State and his executive orders last week to strengthen gun control were flawed and should be changed.

Fiorina, the former Hewlett-Packard CEO who was demoted from the main stage because of faltering poll numbers, dominated the so-called “undercard” debate with a series of one-line zingers, one of them aimed at Democrat Hillary Clinton over her prior marital strains with Bill Clinton, the former president.

“Unlike another woman in this race, I actually love spending time with my husband,” said Fiorina.

U.S. Senator Paul Rand of Kentucky opted out of the "undercard debate" and said instead he would host a town hall session on Twitter during the main debate.

Front-runner Trump has put Cruz, his main obstacle to an Iowa victory, on the defensive by suggesting Cruz may not qualify to be a candidate because he was born in Canada (to a U.S. citizen mother and a Cuban father).

This was a change in posture by Trump on the issue, however, after saying in an interview with ABC News last September that he had heard that lawyers believed it was not a problem for Cruz.

Trump and Cruz have been friendly over the past year, until now.

Being targeted by Trump is a new position for Cruz, who has taken the lead in some polls of Iowa Republican voters in part by avoiding tangles with Trump.

In the Reuters rolling national poll on Jan. 12, Trump had 39 percent of the vote, Cruz 14.5 percent, Bush 10.6 percent, Carson 9.6 percent and 6.7 percent favored Rubio, once viewed by the Republican establishment and many donors as a rising star.

Cruz told reporters in New Hampshire on Tuesday that Trump is relying on a judgment on the birthright question from Harvard law professor Laurence Tribe, who Cruz described as "a liberal, left-wing, judicial-activist" and a supporter of Democratic Party front-runner Hillary Clinton.

"The past couple of elections we saw the Democrats thrilled that they got the nominee they wanted to run against in the general election, and it seems the Hillary folks are very eager to support Donald Trump," Cruz said.

Trump, who has proved to be a master at finding a perceived weakness in an opponent, insisted Cruz's Canadian birth violated the U.S. Constitution's requirement that only "natural born citizens" can be president.

"Sadly, there is no way that Ted Cruz can continue running in the Republican Primary unless he can erase doubt on eligibility. Dems will sue!" he tweeted on Wednesday.

Mudslinging abounds elsewhere in the field as a grouping of other candidates fight to be the alternative to Trump with battles over national security and immigration.

For more on the 2016 presidential race, see the Reuters blog, “Tales from the Trail” (http://blogs.reuters.com/talesfromthetrail/)

(Reporting By Steve Holland; editing by Grant McCool)