Senate Majority Leader John Thune — and most of the GOP conference — was reeling Tuesday from President Trump's snap endorsement of Ken Paxton in the Texas GOP Senate runoff. Why it matters: Thune (R-S.D.) called it Trump's "decision." But it's Thune's problem to pass Trump's agenda, with a trio of senators the president can no longer hurt.Sen. Thom Tillis (R-N.C.) has been unbowed since announcing his retirement in July 2025 after clashes with Trump.Sen. Bill Cassidy (R-La.) voted with Democrats Wednesday to advance an Iran war powers vote to debate.Sen. John Cornyn (R-Texas) has a runoff May 26 but will enter it with Trump backing someone else.😓 Adding to Thune's degree of difficulty: a president sick of the Senate and its rules, and rank-and-file Republicans seething over Trump's knifing of Cornyn."I don't understand. He [Paxton] is an ethically challenged individual," said Sen. Susan Collins (R-Maine)."I'm supremely disappointed," Sen. Lisa Murkowski (R-Alaska) said.Driving the news: Trump's endorsement of Paxton came in the middle of his 10-day GOP revenge tour.Cassidy fell Saturday, failing to make the runoff in Louisiana's Senate primary."Horrible Congressman Thomas Massie" (R-Ky.), as Trump called him, has his primary today.What we're watching: The $72 billion reconciliation package will be the first test for the Senate's newly combustible environment."I just came off the campaign trail. People love the president," Cassidy said. "But I can tell you, a billion dollars for the ballroom … that's not where their head is, and that's not where my head is."Zoom out: Trump's relations with the Senate have sunk to new lows over his frustration with the Senate parliamentarian, Republicans' refusal to abolish the filibuster, and GOP handwringing over ballroom security funding.Thune pronouncing himself "not a big fan" of the new $1.76 billion Department of Justice "anti-weaponization" fund this morning could have been the final straw.What they're saying: "He's done with the Senate bullsh*t and Thune and all of them. They can't deliver," said a Trump confidante."The only reason the president was holding out for Cornyn was the SAVE Act," the confidante said. "And when that became a lost cause, it was 'Why the f*ck should I support this guy?'"[Trump] saw some polling this weekend.… The polling showed Paxton will win in November. It might have been Paxton's polling. But it's Texas," a Trump adviser said.⚡️ Zoom in: GOP lawmakers have several must-pass bills in the next five months, followed by several nice-to-pass pieces of legislation.Trump is demanding money for ICE and Border Patrol before June 1. Section 702 of the Foreign Intelligence Surveillance Act expires June 12. The farm bill needs to pass by Sept. 30.Republicans are working to pass a housing bill to lower costs and looking for 60 votes on a bill to provide market structure for cryptocurrency.A reconciliation 3.0 package is still being discussed — with varying degrees of earnestness and snickering — in both chambers.🙀 The bottom line: Trump has always enforced GOP loyalty with raw fear, naked threats and decisive punishment.Now he'll learn whether fear loses its hold on lawmakers who have already lost — or are staring at defeat.