North Charleston (United States) (AFP) - With the first vote of America's 2016 election two weeks away, Republican presidential frontrunners Donald Trump and Ted Cruz dispensed with months of niceties and locked horns in a primetime debate.

After months of chaotic campaigning that saw a dozen long-shot candidates vie for attention, the debate in South Carolina brought the race to become the Republican presidential nominee into sharper focus.

Trump and Cruz, who lead the polls in Iowa -- which on February 1 will become the first state in the country to vote for the nominee -- at times seemed to ignore the rest of the field as they traded blows.

The duo went back-and-forth over conservative values and Trump's allegation that Cruz, a hardline Texas Senator born in Canada, might be legally blocked from becoming president.

Cruz accused the controversial mogul of manufacturing a crisis to retain his lead.

"Back in September, my friend Donald said he had his lawyers look at this from every which way and there was no issue there," said Cruz.

"Now since September, the constitution hasn't changed. But the poll numbers have," he added. "Donald is dismayed that his poll numbers are falling in Iowa."

He went on to point out that Trump, whose mother was born in Scotland, was disqualifying himself.

Trump shot back: "As you know, Ted, in the last three polls I'm beating you. So you shouldn't misrepresent how well you're doing with the polls."

"You have a big lawsuit over your head."

- Unwelcome frontrunner -

The debate among seven Republican candidates came as unease grows within the Republican party over Trump's frontrunner status.

The controversial celebrity has long led in national polls and his campaign shows no signs of collapsing before Iowa becomes the first state to vote in a long nominating process.

Trump has ridden a wave of populist anger with Washington, frustration over the nation's patchy economic recovery, and fear about a growing terrorism threat.

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After months of tip-toeing around the frontrunner, the party establishment appears to be mustering its forces, believing his inflammatory message is more dangerous than the risk of him launching an independent run.

South Carolina's own Governor Nikki Haley fired the opening salvo against Trump during her rebuttal to President Barack Obama's State of the Union address Tuesday, calling on Republicans to ignore the "angriest voices" in their party.

Trump described Haley as a "friend," but the criticism brought into the open a struggle between a rebellious rank-and-file drawn to Trump's outsider populism and the party's conservative establishment.

In a clear play for votes in conservative Iowa, Cruz rounded on Trump, accusing him of having "New York values" -- pro-abortion, pro-gay rights and pro media.

"Not a lot of conservatives come out of Manhattan," he joked.

Trump described the comment as "very insulting" and defended New Yorkers for how well they bounced back after 9/11.

"When the World Trade Center came down, I saw something that no place on Earth could have handled more beautifully."

Cruz, who has run as an ultra-patriotic anti-elitist, was also forced to address a potentially damaging charge that he borrowed money from Wall Street banks while running for the Senate without properly reporting it.

He admitted to making an error of paperwork, and tried to deflect criticism by accusing the New York Times -- who revealed the non-disclosure -- of conducting a "hit job."

Trump and Cruz were joined on stage Thursday by Senator Marco Rubio; retired neurosurgeon Ben Carson; New Jersey Governor Chris Christie; Ohio Governor John Kasich and former Florida governor Jeb Bush.

Iran's seizure and quick release of 10 US Navy sailors, was another major talking point among the candidates, who lined up to paint Obama as weak and delusional on the topic of national security.

Cruz proclaimed himself "horrified" to see "ten American sailors on their knees with their hands on their head. It was heart-breaking."

Likely Democratic nominee Hillary Clinton also came in for criticism.

Bush, an early favorite who is struggling to make his mark on the race, suggested an investigation into her use of a private server while secretary of state may cause her problems.

If elected, "she might be going back and forth between the White House and the court house," he said.

Rubio, who is seen as the best hope of mainstream conservatives, went further saying "she wouldn't just be a disaster, Hillary Clinton is disqualified from being commander-in-chief of the United States."