A scenario in which Trump is sent home to Mar-a-Lago while Republicans retain control of the upper chamber would be not only palatable, the aide said, but maybe even preferable.

And yet, that’s not likely to happen. Trump’s fate is yoked to that of Senate Republicans, whose fragile majority likely depends on his reelection. Put simply, vulnerable Republican senators are in a bind: They need to win the backing of Trump’s enthusiastic base without driving away more independent voters who are an important part of a winning coalition. They want the energy Trump creates, but not the divisiveness. What the past four years have shown is, you don’t get one without the other.

Trump has made the presidency about himself, and now the same is largely true for Senate races. Splitting the ticket—voting for one party in the presidential race and another in down-ballot contests—has grown vanishingly rare in recent decades. Every state with a Senate election that Trump carried in 2016 also chose the Republican candidate, while each state with a Senate election that Hillary Clinton won picked the Democrat. That pattern should hold in 2020, reinforced by Trump’s ubiquity. “Everyone puts most candidates in two buckets: pro-Trump or anti-Trump. That’s it,” a former senior White House official told me.