Republicans lost both Senate races in Georgia on Tuesday, losing control of the Upper Chamber, and you don’t have to be Karl Rove to understand what happened.

If you want to win, don’t tell voters that their votes don’t matter. Don’t have a President tell his voters that the last election was stolen, that Georgia Republicans were complicit in the theft, that GOP Senate leaders don't care, and then expect those same voters to turn out in the Senate runoffs after a rally and a few tweets.

Donald Trump cost Republicans the Senate by making the two Georgia runoffs a referendum on himself. The races should have been a referendum on blocking Democrats from controlling all of Congress and the executive branch. But that message was obscured, if not obliterated, by Mr. Trump’s insistence on telling voters day after day that he was cheated in November—no matter the lack of credible evidence or plausible path to victory.

(The editorial goes on to describe what will be lost without a divided government.)