Donald Trump says he would tell Pope Francis "ISIS wants to get you," if the pontiff criticised capitalism during his visit to the US next month.

"I'm gonna have to scare the Pope because it's the only thing," the Republican frontrunner told CNN last night.

"The Pope, I hope, can only be scared by God. But the truth is - you know, if you look at what's going on - they better hope that capitalism works, because it's the only thing we have right now. And it's a great thing when it works properly."

Capitalism has not been working properly in the US, Trump said, because of over regulation on industry.

"We are not as competitive as we used to be," he said. "We have to open up our country to great capitalism."

The pontiff's advisers have reportedly urged him to tone down his critique of capitalism to make him more palatable to American audiences. Pope Francis has recently described unfettered capitalism as "the dung of the devil."

Trump is favourite to be the Republican presidential candidate, according to the latest polls. Despite his threat and their evident ideological differences, the outspoken real estate mogul said Pope Francis seemed "like a pretty good guy."

"I have great respect for the Pope," Trump said. "I like the Pope. I actually like him. He's becoming very political, there's no question about it. But I like him. He seems like a pretty good guy."