Donald Trump insists, as president, he will make Mexico pay for a wall on the southern U.S. border to help prevent undocumented immigrants from crossing into America.

Trump told Fox News' Bill O'Reilly on Tuesday he is sure he can get Mexico to pay for the wall despite financing being an issue approaching the 2016 presidential elections.

"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 asked him the question twice.

When asked a third time the billionaire businessman 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 is getting ready for a debate among several other Republican presidential hopefuls on Thursday and insists he is not nervous.

"The biggest thing in my life is my family and my children," he said. "This is a different kind of 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."

The businessman also said he believes Republicans should shut down the government rather than fund Planned Parenthood after a video leaked that showed PPH officials discussing selling fetal tissue and organs for medical research. There was a vote to defund the organization from federal aid on Tuesday in the Senate, but opponents failed to get the votes needed to pass a procedural hurdle.

Meanwhile, reports show Trump has taken credit for charitable gifts made by others and has donated considerably less to charity than others in his tax bracket.

The first Republican presidential debate is scheduled for 9 p.m. EST.