For months, Mr. Obama had kept in close contact with senior party officials, in hopes of preventing a repeat of the protracted and nasty 2016 primary race.

Then, in the weeks after it became clear that Mr. Biden was the party’s near-certain nominee, Mr. Obama — telling a friend he needed to “accelerate the endgame” — had at least four long conversations with his former vice president’s remaining rival, Senator Bernie Sanders. Mr. Obama’s efforts to ease the senator out of the race played a significant role in Mr. Sanders’s decision to end his bid and endorse Mr. Biden, according to people close to the Vermont independent…

Mr. Obama did not directly encourage Mr. Sanders’s rivals to endorse Mr. Biden ahead of the decisive Super Tuesday primaries. But he did tell Pete Buttigieg, a moderate, that he would never have more leverage than on the day that he was quitting the race — and the former South Bend mayor soon joined the avalanche of former candidates backing Mr. Biden.