While the battle for the Democratic presidential nomination was on, Barack Obama remained mute. Only on Tuesday, the day after Bernie Sanders had conceded and endorsed Joe Biden, did Mr. Obama re-enter the fray with a video endorsement of Mr. Biden.

It struck some observers as odd that the former president didn’t endorse his old running mate and vice president earlier. But Mr. Obama’s decision to wait was both to be expected and politically smart. There was concern among the Obama political team about Mr. Biden’s effectiveness...