Today, Joe Biden will quietly ring in his 80th birthday over brunch with his family in Washington. It’s a milestone none of his predecessors reached while serving in the White House and one that looms large as he considers his political future. Yet the president enters his ninth decade at a moment of unexpected strength. Democrats defied history in the midterm elections, keeping control of the Senate and shattering Republican hopes of a “red wave” in the House. The verdict – a beat-the-odds performance by his party and the defeat of several election-denying candidates backed by Donald Trump – offered...