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Speaking in Salt Lake City — home to the Church of Jesus Christ of Latter-day Saints’ headquarters — Donald J. Trump questioned Mitt Romney’s membership in the faith on Friday, asking a crowd at a rally, “Are you sure he’s a Mormon?”

Mr. Romney, who was the Republican presidential nominee in 2012, comes from a prominent Mormon family, and he remains popular in Utah, which has a sizable Mormon population.

After first praising Mormons generally — “And do I love the Mormons, O.K.?” Mr. Trump said. “Do I love the Mormons?” — Mr. Trump added that he has many friends in Salt Lake City. But he noted pointedly that Mr. Romney, who has been urging Republicans to deny him the presidential nomination, is not one of them.

“Did he choke? Did this guy choke? He’s a choke artist,” Mr. Trump said, referring to Mr. Romney’s loss to President Obama in 2012. Mr. Trump added, “Are you sure he’s a Mormon? Are we sure?”

Though Mr. Trump endorsed Mr. Romney in 2012, the two men are far from friendly now. Earlier this month, Mr. Romney delivered a scathing broadside against Mr. Trump, calling him “a fraud” and “a phony” and urging the Republican Party to unite around an alternative candidate.

And on Friday, Mr. Romney, who owns a home in Holladay, Utah, wrote in a Facebook message that he planned to vote for Senator Ted Cruz of Texas in the state’s Republican caucus Tuesday, as part of an effort to deny Mr. Trump the party’s nomination at its convention in July.

Mr. Trump has questioned the religious affiliation of his rivals before, including Ben Carson, the retired neurosurgeon, who is a Seventh-Day Adventist. But after Pope Francis recently suggested that Mr. Trump was not Christian because of his promise to build a wall along the Mexican border, the real estate mogul took offense, saying it was “disgraceful” for a religious leader to question someone else’s faith.

In his remarks Friday evening, Mr. Trump also criticized Mr. Cruz. “Is he really a natural born citizen? I mean, give me a break,” Mr. Trump said, referring to Mr. Cruz, who was born in Canada and recently renounced his Canadian citizenship. “He was a citizen of Canada, can you believe it?

“He was a joint — he was U.S. and he was Canada,” Mr. Trump continued. “And you know, that’s not the way it’s supposed to work. You’re supposed to be born, like, here.”

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