A common feature of Donald Trump’s two terms as President is that he can’t stand political prosperity. When events are going in his direction, he has an uncanny habit of handing his opponents a sword.

The latest example came Sunday after GOP leaders dragged several holdouts across the line to vote aye and start debate over the Senate version of the budget reconciliation bill. The vote was 51-49, but Mr. Trump couldn’t leave victory alone.

Instead he unloaded on the two GOP dissenters, Rand Paul of Kentucky and especially Thom Tillis of North Carolina. Mr. Trump dumped three different Truth Social posts on Mr. Tillis, calling him a sellout, a “talker and complainer” and various other insults.

He also all but promised to support a Republican opponent in the GOP primary for Senate in 2026, when Mr. Tillis’s term ends. “Numerous people have come forward wanting to run in the Primary against ‘Senator Thom’ Tillis,” Mr. Trump said on Truth Social. “I will be meeting with them over the coming weeks, looking for someone who will properly represent the Great People of North Carolina.”

Mr. Tillis promptly said he won’t run for re-election. Even if Mr. Tillis had already been contemplating retirement, his withdrawal opens a seat that is another pickup opportunity for Democrats next year.

The GOP has a 53-47 majority now, but Susan Collins always has a tough race in Maine if she decides to run again. Democrats are targeting Joni Ernst in Iowa. In the suicide-isn’t-painless department, Texas Attorney General Ken Paxton is challenging GOP incumbent Sen. John Cornyn. Mr. Paxton may be the only Republican who could lose in Texas given his record harassing business with lawsuits, impeachment, and other embarrassments.

The GOP pickup opportunities are few, so with Mr. Tillis’s departure the Senate is in play for 2026. Oh, and on Saturday GOP Rep. Don Bacon said he won’t run for re-election in his swing Omaha seat. That’s a likely gain for Democrats in the House.

Messrs. Tillis and Bacon didn’t help themselves by echoing Democratic attacks against the GOP’s very modest Medicaid changes. But then Mr. Trump and GOP leaders haven’t helped them or the party by failing to make the moral and fiscal case for those reforms.

GOP legislative reforms will have no chance if Democrats take the House in 2026. And if they also take the Senate, forget about confirming another Supreme Court nominee. The Trump Presidency will be dead in the water.