In conversations late into the evening, Mr. Trump repeatedly hesitated over selecting Mr. Pence, according to people briefed on the tense deliberations, who insisted on anonymity to describe the confidential talks. Even as his emissaries reassured Mr. Pence, Mr. Trump fielded a last-ditch appeal from Gov. Chris Christie of New Jersey, another finalist, who once again pressed his own case.

Advisers and family members stressed over and over to Mr. Trump that he was selecting a running mate to unite the Republican Party, not a new best friend.

But before flying back from a fund-raiser in California, Mr. Trump had settled on a plan for the morning. After having insisted that he would not make a Friday announcement out of respect for the attack in Nice, France, he would reverse course, appropriately enough, on Twitter, and name Mr. Pence as his choice…

Reached by phone on Friday, Mr. Manafort said Mr. Trump had never wavered on Mr. Pence once the decision was made. Any queries about Mr. Pence as a political choice, he added, simply reflected Mr. Trump’s loose conversational style.