U.S. Republican presidential front-runner Donald Trump is not pleased with Pope Francis' plan to stand with migrants at the U.S.-Mexican border on Friday, saying the head of the Roman Catholic Church "doesn't understand" America's immigration problem.

Speaking to Fox Business Network on Thursday as reported by Newsmax, Trump described Pope Francis as a "very political person" as the pontiff is set to begin his first official visit to Mexico on Friday.

"I don't think he understands the danger of the open border that we have with Mexico, and I think Mexico got him to do it. Because Mexico wants to keep the border just the way it is because they are making a fortune and we are losing," Trump said.

Trump once expressed admiration for the 79-year-old pontiff, even suggesting he would have liked to have had him on his reality TV show "Celebrity Apprentice," CBN News reported.

However, in August, Trump said in case he gets a chance to meet with Pope Francis, he would warn him about the danger of the Islamic State (ISIS).

He said he would tell the pope, "You know that ISIS wants to go in and take over the Vatican? You have heard that. You know, that's a dream of theirs, to go into Italy."

Meanwhile, Trump has been accused by Texas Senator Ted Cruz, his closest rival for the GOP presidential nomination, of having shown a "pattern of sleaze going back decades."

The accusation is contained in Cruz's latest TV spot which began airing on Thursday, Newsmax reported.

On the same day, Trump accused Cruz of using "sleazy and dishonest" tactics in South Carolina.

GOP consultant Karl Rove, meanwhile, urged four Republican candidates—Cruz, Marco Rubio, John Kasich, and Jeb Bush—to sharply step up their attacks on the front-running Trump instead of fighting each other, or else "the chances of nominating a mainstream Republican may dissipate."

"Messrs. Kasich, Cruz, Bush and Rubio must resist the temptation to go after one another — which only wastes vital time — and instead concentrate fire on Mr. Trump," Rove wrote Wednesday in an op-ed piece in The Wall Street Journal as reported by Newsmax.

Rove said the four GOP bets should hammer Trump on his "greatest weakness: Americans have never elected a serial bankrupt."

The four bets should impress upon the voters that when Trump's companies went under, "such a wealthy man didn't dip into his fortune to do right by the people who were hurt," Rove said.

He said they should also pounce on Trump's "claim to be a great businessman," noting that the billionaire's "casinos never reported a profit" and that "the only person who may have made big money on them was The Donald, when he sold."

"So far Mr. Trump's response to the bankruptcy charge has been that he 'took advantage of the laws,'" Rove said.

"Thoroughly airing the issue will provide an opportunity for him to give a better answer — or for Republicans to decide they don't want a nominee with such baggage."