If you are a Republican on the ballot, you are in the same boat as Mr. Trump, whether you like it or not. If enough people jump ship, generic party identification will suffer, and everyone will sink. In other words, if enough Republicans run from their leader, the Republican brand will be so diminished as to produce historic defeats up and down the ballot.

This happened to the Democrats in 2010. To their credit, the Democratic consultants and members of Congress never formed a professional class of Never Obama Democrats. They stayed in the boat. But when it came to defending Obamacare, they stopped rowing. The lesson for Republicans: Stay in the boat and keep rowing.

No, you don’t have to support the president’s tweet storms. But you do have to defend his policy accomplishments. If the election is only a personality contest — rather than about the president’s policies — a historic defeat awaits.