Donald Trump and Ted Cruz lashed into one another over the Texas senator’s Canadian birth and the billionaire’s “New York values” at the Republican Party’s sixth presidential debate Thursday night as the two perceived front-runners for the upcoming Iowa caucus fought for an edge.

In another fierce battle in the debate in North Charleston, South Carolina, New Jersey Gov. Chris Christie accused Sen. Marco Rubio of Florida of lying about his record to build his status with mainstream Republican supporters. Cruz and Rubio tried to paint each other as soft on immigration. Former Florida Gov. Jeb Bush and Ohio Gov. John Kasich sought to appear as veteran leaders in a field of inexperienced hotheads.

The Republican field repeatedly attacked President Barack Obama and Hillary Clinton, the Democratic front-runner. They accused Obama of trying to confiscate guns and downplaying the threat of ISIS and bashed Clinton’s foreign-affairs and tax policies.

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But the most significant brawl took place between Trump and Cruz, who largely exchanged pleasantries in previous GOP debates. This time, they clashed harshly and repeatedly over Cruz’s constitutional eligibility for the presidency and his attempt to discredit Trump among Republicans by saying the developer had “New York values.”

Invoking New Yorkers’ response to the 9/11 attacks, Trump said Cruz had “insulted a lot of people.”

Cruz, who was born in Canada to an American mother and a Cuban father, said Trump raised a phony issue of whether he met the test of being a “natural-born citizen” because of tightening polls.

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“I recognize that Donald is dismayed that his poll numbers are falling,” Cruz said, noting that 2008 GOP presidential candidate John McCain, from a U.S. military family, was born in Panama. He pointed out that Trump’s mother was Scottish.

“I was born here. Big difference,” Trump said, who segued to saying that if he chose Cruz to be his vice presidential running mate, no one would know whether Cruz could legally serve.

“There’s a big question mark over your head, and you can’t do that to the party,” Trump said. “I’m not bringing a suit, I promise. But the Democrats will sue.”

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Cruz, saying as a lawyer he’s spent time arguing the U.S. Constitution in court, fired back: “I’m not going to be taking legal advice from Donald Trump.”

Cruz also was put on the spot about his characterization of Trump having “New York values.”

“I think everyone knows what New York values are,” Cruz said, decrying “socially liberal” policies. Cruz, whose wife works for New York-based Goldman Sachs, added: “Not a lot of conservatives come out of Manhattan.”

“He insulted a lot of people,” Trump replied, adding that “when the World Trade Center came down,” no one could have handled it more “beautifully” than New Yorkers.

Trump was asked about the comments of South Carolina Gov. Nikki Haley, who recently said Trump was appealing to voters’ anger. “I will gladly accept the mantle of anger,” Trump said.

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Bush and Kasich tried to portray the others onstage as lacking in judgment and experience — especially in foreign affairs.

“It takes strength. You’ve got to be cool. But you got to know where you are going,” Kasich said, adding one can’t rely on “on-the-job training.”

Kasich said he’d heard a lot of “hot” rhetoric in the debate meant to appeal to angry GOP primary voters but said, referring to the general election, “at the end of the day, the country needs to heal.”

Bush dismissed Cruz and Rubio as “backbench senators” and said Republicans need a “compelling, compassionate agenda that doesn’t disparage people and unites us in a common” goal.

Bush ridiculed Trump’s call for a ban on all Muslims entering the United States as harmful to U.S. efforts to win allies in the Islamic world. Trump then called Bush “weak.”

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Rubio got into it with Christie, calling him too liberal, saying Christie “wrote a check to Planned Parenthood” and supported gun control and Sonia Sotomayor, one of Obama’s Supreme Court nominees.

That fired up Christie, who said Rubio thought he could gain in the race by knocking the New Jersey governor.

“Two years ago, he called me a conservative reformer New Jersey needed. Now, he’s changed his tune,” Christie said.

Later, when Rubio tried to cut into Christie’s time on a separate question, Christie pulled Rubio up short: “No, you already had your chance, Marco, and you blew it.”

Rubio and Cruz battled late in the forum over tax plans and immigration. Cruz accused Rubio of wanting to ease immigration and of supporting proposals by Sen. Charles Schumer (D-N.Y.). Rubio said Cruz flipped on increasing the volume of green cards, among other things.

Retired neurosurgeon Ben Carson largely was a noncombatant. When he did jump in, it mainly was to attack Obama. Asked about the president’s view, expressed in the State of the Union address, that ISIS wasn’t a serious threat to American democracy like the United States’ World War II enemies, Carson said the president was stuck in the 20th century.

“War is very different than it used to be before,” Carson said. “We now we have dirty bombs and cyberattacks and people who will be attacking our electrical grid.”