Washington CNN  — 

Donald Trump – who Fox News said literally will be at the center of its GOP primary debate on Thursday – is already feeling the heat from network hosts.

On Tuesday, Fox’s Bill O’Reilly grilled the billionaire businessman on his claim that as president he will get Mexico to pay for a wall on the southern U.S. border to help prevent undocumented immigrants from crossing into the United States.

“Bill, they are making a fortune, Mexico is making a fortune off the United States, it’s becoming the new China in terms of trade – they’re killing us at the border,” Trump said after O’Reilly pressed him twice on the same question.

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The third time O’Reilly asked, Trump said, “I’m gonna say, ‘Mexico, this is not going to continue, you’re going to pay for that wall,’ and they will pay for the wall. And Bill, it’s peanuts, what we’re talking about.”

Trump has been critical of Ford Motor Company for a proposed $2.5 billion Mexican plant and has said under a Trump presidency he would impose a tax on Ford parts imported from there to also subsidize the wall.

“They’re taking in billions and billions of dollars, the Ford Company,” Trump told O’Reilly.

He also swatted away the idea that he should be nervous for Thursday night’s showdown.

“The biggest thing in my life is my family and my children,” he said. “This is different kind of a thing. This is a big league deal, there’s no question about it … But I’m doing it because it’s something you have to do.”

READ: What would a Donald Trump administration look like?