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Rudolph W. Giuliani is not endorsing Donald J. Trump. But he is voting for him.

In an interview, Mr. Giuliani, the former mayor of New York who has been uncharacteristically quiet in the last week about the Republican primary, said that he was planning to vote for Mr. Trump in the state’s primary on April 19, over Senator Ted Cruz of Texas and Gov. John Kasich of Ohio.

“He’s my friend, and I think of the candidates, of the choices that I have, he’s the best choice for president,” Mr. Giuliani said. “He’s a better choice than Cruz and a more realistic choice than Kasich.”

Mr. Giuliani, who will be a delegate from Manhattan at the Republican National Convention, said he would have more sway on the floor as someone who had not endorsed anyone. But he spoke out as speculation swirled about why he had been absent in the lead-up to the primary in his home state.

Mr. Giuliani said he had been traveling, but that on several — not all — areas of policy, he agreed with Mr. Trump.

He acknowledged that he did not care for Mr. Trump’s Twitter post about Mr. Cruz’s wife, Heidi, which featured an unflattering picture of her.

“I didn’t like that,” Mr. Giuliani said. He also said he did not like the use of a photo of Mr. Trump’s wife, Melania, by a “super PAC” that is working to defeat him. “He’s a first-time candidate — you’re going to have some of these mistakes,” Mr. Giuliani said. “The man that I know is not the man you see on television. He’s a gentleman, he’s a good father.”

He added that there was never a plan to meet with Mr. Cruz, despite a report that such a sit-down was in the offing.

Mr. Trump, who had hoped for weeks for a formal endorsement from his friend of three decades, called Mr. Giuliani after his interview with The Times. The former mayor explained his logic in why he was not making a formal endorsement, according to a person briefed on the call, who was not authorized to speak publicly about it.

In the Times interview, Mr. Giuliani said that he doesn’t agree with either Mr. Trump or Mr. Cruz on mass deportations, and said he has his own plan for immigration reform that he has carried around in his briefcase for several years.

But he said he agreed with Mr. Trump in other areas, and compared it to the situation with President Ronald Reagan, under whom he served at the Justice Department. He agreed with Mr. Reagan the majority of the time, but not all of it, Mr. Giuliani said.