Republican presidential front-runner Donald Trump and top challenger Ted Cruz clashed sharply on Thursday over the Texas senator’s eligibility to run for the White House during a tense debate weeks before voters start to pick the party’s nominee.

Republican U.S. presidential candidate businessman Donald Trump (L) and Senator Ted Cruz speak simultaneously at the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina, January 14, 2016. REUTERS/Randall Hill

Republican U.S. presidential candidate businessman Donald Trump (L) speaks as rival candidate Senator Ted Cruz looks on during the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina January 14, 2016. REUTERS/Randall Hill

Republican U.S. presidential candidates Senator Marco Rubio (L) and Senator Ted Cruz (R) listen to businessman Donald Trump during the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina January 14, 2016. REUTERS/Chris Keane

Republican U.S. presidential candidate and former Governor Jeb Bush waves to the audience at the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina, January 14, 2016. REUTERS/Chris Keane

Republican U.S. presidential candidate Senator Ted Cruz gestures towards rival candidate businessman Donald Trump (L) as he speaks at the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina January 14, 2016. REUTERS/Chris Keane

Republican U.S. presidential candidate businessman Donald Trump reacts during the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina January 14, 2016. REUTERS/Chris Keane

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

NORTH CHARLESTON, S.C. — Republican presidential front-runner Donald Trump and top challenger Ted Cruz clashed sharply on Thursday over the Texas senator’s eligibility to run for the White House during a tense debate weeks before voters start to pick the party’s nominee.

Businessman and reality TV star Trump said Cruz’s path to the presidency was in question because he was born in Canada. The Constitution mandates that only “natural born” citizens can become president of the United States.

Cruz, who was born in Calgary, Alberta, to a U.S. citizen mother and a Cuban father, accused Trump of bringing up his birthplace simply because Cruz was leading some polls in Iowa, which holds the first nominating contest on Feb. 1 in the run up to the November general election.

Cruz said Trump, who led the movement questioning whether the Hawaiian-born President Barack Obama was really from the United States, had asked his lawyers to look into the issue of Cruz’s birth in September and concluded there were no issues.

“Since September, the Constitution hasn’t changed, but the poll numbers have,” Cruz said. “And I recognize that Donald is dismayed that his poll numbers are dropping in Iowa, but the facts and the law here are really clear.”

Trump said Democrats would sue if Cruz were on the Republican ticket, putting their party’s chances of winning at risk.

“There’s a big question mark on your head,” Trump told his rival, with whom he has had friendly relations over the past year on the campaign trail. “The Democrats are going to bring a lawsuit.”

He urged Cruz, who is a lawyer, to ask a court to put the question to rest.

“I’m not going to be taking legal advice from Donald Trump,” Cruz retorted.

Trump, who has proven to be a master at finding a perceived weakness in an opponent, has made an increasing issue of Cruz’s Canadian birth. He admitted during the debate that he was doing so because of Cruz’s improving political prospects.

Cruz has steered clear of criticizing Trump during his rise to the top of the national polls, but that ended on Thursday.

In addition to fighting back over his presidential eligibility, Cruz accused Trump of not being a conservative because he was born in New York and still lives there.

“Everyone understands that the values of New York City are socially liberal, are pro-abortion, are pro-gay marriage,” Cruz said.

Trump rebutted Cruz’s comments by referring to the Sept. 11, 2001 attacks. He said New Yorkers came together to support each other after thousands were killed and cleaned up the site of the destroyed 110-story World Trade Center’s twin towers, enduring “the smell of death” for months.

“That was a very insulting statement that Ted made,” Trump said.

TENSIONS HIGH

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.

Florida Senator Marco Rubio, retired neurosurgeon Ben Carson, former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie and Ohio Gov. John Kasich rounded out the top seven candidates ranked by Republican voters on the stage.

A Google snap poll showed viewers believed Trump was winning the debate more than halfway through, with 32.8 percent backing him and 28.7 percent backing Cruz.

Cruz, who has taken the lead in some polls of Iowa Republican voters, did well on social media. Social media monitoring tool Zoomph saw total positive mentions for Cruz climb by more than 150 percent while he sparred with Trump.

Trump is far ahead nationally.

A Reuters/Ipsos rolling national poll on Jan. 12 showed Trump had 39 percent of the vote, Cruz 14.5 percent, Bush 10.6 percent, Carson 9.6 percent, while 6.7 percent favored Rubio.

The other candidates sought to gain traction in the debate with criticism of Obama’s policies on guns and also threw barbs about his State of the Union address on Tuesday.

In that speech, Obama sought to offer a more optimistic vision for America’s future and he singled out Trump, without naming him, for calling on the United States to ban Muslims temporarily from entering the country.

Bush urged Trump to change his views on the issue. Trump, who noted that his poll numbers rose after making that policy proposal, said his mind was made up.

The billionaire also said he would not “care less” about his company, The Trump Organization, if he became president, allowing executives and his children to run it.

Rubio, once viewed by the Republican establishment and many donors as a rising star, said Cruz was a political opportunist who was not a consistent conservative.

Cruz said Rubio had his facts wrong.