NORTH CHARLESTON, SC — The much-anticipated Donald Trump vs. Ted Cruz showdown took a few minutes to materialize at the Republican presidential debate here — but when it did, it packed a punch.

Cruz forcefully responded to Trump’s accusations that he’s not eligible to be president because he was born in Canada.

“Back in September, my friend Donald said he had his lawyers look at this in every which way,” Cruz said at tonight’s GOP debate. “There was nothing to this birther issue.”

He added: “Since September, the Constitution hasn’t changed. But the poll numbers have.”

While there has been plenty of animosity between Trump and most of his rivals, the billionaire businessman and Cruz have been on largely friendly territory for much of the campaign season. That’s changed, however, as the polls in Iowa tighten ahead of the February 1 caucuses.

Trump has repeatedly questioned Cruz’ eligibility to become president, seeking to eat into the senator’s support in Iowa. Cruz insists he meets the constitutional requirements to serve because he’s a “natural-born” citizen with an American mother.

But in his attack against Trump on Thursday, Cruz noted that some of the more extreme theories on the topic conclude that someone can only become president if both parents were born in the United States. Under that standard, Cruz noted, Trump himself would be ineligible for the presidents because his mother was born in Scotland.”

“On the issue of citizenship, Donald, I’m not going to use your mother’s birth against you,” Cruz said at the debate sponsored by the Fox Business Network.

Trump shot back: “Because it wouldn’t work.”

Cruz also had sharp words for The New York Times for publishing a story this week that said Cruz failed to properly disclose large loans he received during his Senate campaign.

Asked about the controversy, Cruz quickly retorted: “Thank you for passing on that hit piece on the front page of The New York Times.”

“The entire New York Times attack is that I disclosed that loan on one filing with the United States Senate … but it was not in the second filing with the FEC,” Cruz said. “Yes, I made paperwork errors … but if that’s the best hit the New York Times has got, they better go back to the well.”

Cruz and Trump are joined on the prime-time stage by Marco Rubio, Jeb Bush, Chris Christie and John Kasich. Pressure is mounting for someone from the rest of the field to emerge as the clear alternative to Trump and Cruz.

The candidates used the opening moments of their debate here to take shots at President Barack Obama — not one another.

Asked about the state of the economy, Cruz instead answered by criticizing Obama for not mentioning in his State of the Union address the 10 U.S. sailors who were captured by Iran earlier this week.

“Today, many of us picked up our newspapers and we were horrified to see the sight of 10 American sailors on their knees,” Cruz said. When he is president, he added, “No serviceman or any woman will be forced to be on their knees.”

Bush, meanwhile, said the idea that the country is better off today than before Obama took office is an “alternative universe.” Pointing to the growing threat of ISIS — and slamming Obama for referring to the group as the “JV team,” Bush said the President was “missing the whole point” on America’s role in the world.

“For the life of me, I have no understanding why the President thinks everything is going on,” he said.

And Rubio blasted Obama for presiding over an “arrogant” foreign policy.

Earlier Thursday, Carly Fiorina dominated a smaller, less-glamorous debate stage when she joined two other low-polling candidates to discuss everything from national security to technology and Hillary Clinton.

Fiorina’s candidacy has largely been defined by memorable debate performances. And even though she was dropped from the prime-time stage at the debate, she still delivered. Right out of the gate, she dealt a sharp personal attack on Clinton, the Democratic front-runner.

“I’m not a political insider. I haven’t spent my lifetime running for office,” the former Hewlett-Packard CEO said. “And unlike another woman in this race, I actually love spending time with my husband.”

Minutes later, Fiorina trained her fire on Trump.

“Despite Donald Trump’s bromance with Vladimir Putin … Russia is our adversary,” she said.

Mike Huckabee and Rick Santorum joined Fiorina at the earlier debate here. All three of the candidates were sharply critical of Obama on issues such as gun control, national security and the economy.

“We have a president who seems to be more interested in protecting the reputation and image of Islam than protecting us,” Huckabee said.

Santorum, whose core base includes blue-collar conservatives, slammed the President on job creation.

“The numbers just don’t add up,” he said. Obama “has done more to take jobs away from the hard-working people who are struggling the most.”

Rand Paul, who also didn’t qualify for the main debate, skipped the event.

“People have to realize that what the media is doing here is predeciding an election,” Paul said on CNN’s “New Day.”