President Donald Trump proved again Saturday that he can crush Republican dissenters by helping to defeat Sen. Bill Cassidy in a Louisiana primary. The question is to what end?

Mr. Cassidy earned Mr. Trump’s eternal enmity when he was one of seven GOP Senators who voted to convict him after he was impeached a second time in 2021 after the Jan. 6 Capitol riot. It was a vote of conscience for Mr. Cassidy, but fealty counts for more than principle in the Trump universe.

Mr. Cassidy tried to earn back Mr. Trump’s support by overcoming the senator’s misgivings and voting to confirm Robert F. Kennedy Jr. as Health and Human Services Secretary. But Mr. Trump was never going to forget, much less forgive.

The Louisiana GOP, led by Gov. Jeff Landry, helped seal Mr. Cassidy’s fate by passing a law that changed the state’s electoral process from an open primary into closed partisan primaries. Mr. Cassidy has traditionally had support among independents and Democrats. Louisiana is now the reverse partisan image of Democratic New York in restricting voter choice.

Louisiana is a conservative state, so whoever wins the GOP runoff between Rep. Julia Letlow (Mr. Trump’s favorite) and state treasurer John Fleming is likely to hold the seat in November. But Mr. Trump may find he’s now liberated Mr. Cassidy, who can vote as he pleases this year without fear of further retribution. Watch what happens if Mr. Trump tries to nominate Jeanine Pirro as Attorney General.

Mr. Trump is desperate to hold off the day when he is seen is a lame duck, but what matters more than his sway over the GOP is his overall approval rating. At 40.1% in the RealClearPolitics polling average, that rating puts Republican control of the House and Senate in serious jeopardy.

Mr. Trump’s revenge campaign has already made the Senate harder to hold. He drove incumbent Thom Tillis into retirement in North Carolina, and Democrats have a strong candidate who is now the favorite.

Mr. Trump has said Maine Sen. Susan Collins should “never be elected to office again” because of a vote on war powers that had no chance of becoming law and that could hurt GOP turnout for her in a difficult state for Republicans. MAGA threats may also have contributed to Sen. Joni Ernst’s retirement in Iowa, and that seat is now a possible Democrat pickup as the farm economy struggles amid tariffs and inflation.

Then there’s Mr. Trump’s refusal to endorse Sen. John Cornyn in the Texas primary, despite pleas by Majority Leader John Thune and campaign chairman Tim Scott. That opens the door to a possible primary win by Texas Attorney General Ken Paxton, who divides the GOP and could lose to Democrat James Talarico.

Losing all of those seats would take the GOP down to 49 seats, barring other partisan change in other states. If Republicans lose the House, the President’s legislative agenda is effectively over. If they lose the Senate, his ability to nominate conservative judges will be done. He’ll spend two years defending himself against nonstop Democratic investigations into his family businesses, and so much more.

Mr. Trump has dominated GOP fundraising since his re-election. He’d better be ready to spend it all if he doesn’t want to spend his final two years as a really lame duck.

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