Back when few people took Donald Trump seriously as a potential presidential candidate, the New York businessman asked former House Speaker Newt Gingrich and his wife, Callista, to meet in Iowa. Over breakfast at the Des Moines Marriott Hotel in January 2015, Mr. Trump spent 45 minutes grilling Mr. Gingrich on his experience running for president.

“It was clear to me at the end of the talk that he was seriously considering it,” Mr. Gingrich said.

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