Joe Biden is no more or less capable a president than he was two months ago. His staff is no more or less competent. But suddenly, images of Biden as a feeble septuagenarian atop a mismanaged White House have given way to those of an experienced leader, smiling behind aviator sunglasses, whose battle-tested team has delivered on a range of national priorities. A winning streak does that for you. It has not happened because of a strategy shift or staff shakeup, though at low points allies wanted him to take those ritual steps. It’s been a combination of good luck,...