Republican senators reacted angrily on Tuesday to President Trump’s decision to endorse Ken Paxton, the Texas attorney general, in the state’s Senate primary runoff, warning that his snub of the incumbent Senator John Cornyn could risk the seat and the party’s fight to keep its majority.
Heading into their weekly G.O.P. luncheon not long after Mr. Trump posted his choice on social media, many Senate Republicans appeared stunned and livid as they learned the news, which dealt a serious blow to Mr. Cornyn, who has served for more than two decades.
“Oh boy,” said a visibly dismayed Senator John Hoeven of North Dakota, as he left the Senate chamber after hearing of Mr. Trump’s announcement.
“Well, obviously,” he added, “I support Senator Cornyn.”
A stone-faced Senator Roger Wicker of Mississippi refused to answer questions about the endorsement as he exited the chamber following a vote.
Senator Lisa Murkowski, the Alaska Republican whose independent streak has often angered the president, said that she was “supremely disappointed” by Mr. Trump’s decision.
Then she went a step further, saying that the president’s endorsement of Mr. Paxton, a scandal-plagued conservative firebrand, could cost Republicans what had been considered a safe Senate seat.
“I think that this puts that seat in jeopardy,” she told reporters.
Mr. Trump’s endorsement, which came as early voting is underway in Texas, is expected to boost Mr. Paxton’s odds in what has already been an expensive and bitter election.
Many Senate Republicans, including the majority leader, John Thune of South Dakota, had been urging Mr. Trump to back Mr. Cornyn, whom they saw as a stronger candidate in a general election. The president’s decision to do otherwise amounted to a slap at Mr. Thune, an institutionalist like Mr. Cornyn.
“It’s his decision,” Mr. Thune told reporters of Mr. Trump as he entered the party luncheon at the Capitol, throwing up a hand in a gesture of exasperation.
“None of us control what the president does,” Mr. Thune said later. “He made his decision. That doesn’t change the way I feel.”
The endorsement was also likely to further sour an already tense relationship between Mr. Trump and some Republican senators who have bristled at his tactics and decisions during his second term, as the G.O.P. has deferred to him rather than insisting on the Senate’s tradition of independence.
With just a narrow majority, Senate leaders already faced challenges muscling through many of the president’s priorities.
But they are now worried that the defeat of Senator Bill Cassidy of Louisiana by a Trump-backed challenger last week and the president’s rejection of Mr. Cornyn will further complicate their efforts to pass consequential legislation, according to two leadership aides who requested anonymity to discuss internal political considerations.
Both Mr. Cassidy and Mr. Cornyn had engaged in political contortions to appease Mr. Trump and win his endorsement. But just days after his loss, Mr. Cassidy, whose 2021 vote to convict Mr. Trump of inciting an insurrection precipitated his break from the president, has displayed a more independent streak.
On Tuesday, Mr. Cassidy, who was greeted with a loud standing ovation from his fellow Republicans inside the luncheon, said that he would vote against a bill to fund immigration enforcement operations if it included security money tied to Mr. Trump’s White House ballroom project.
If Mr. Cornyn were to lose his primary, Mr. Trump would face the prospect of at least three lame duck senators more willing to buck his demands. Senator Thom Tillis, Republican of North Carolina, decided to retire last year after Mr. Trump threatened to back a challenger.
Senator Susan Collins, a Maine Republican who is running for re-election and has also been a thorn in Mr. Trump’s side, also said she was dismayed about Mr. Trump’s endorsement of Mr. Paxton.
“John Cornyn is an outstanding senator and deserved, in my judgment, the president’s support,” Ms. Collins said. “Obviously, it’s the president’s call, but I’m disappointed that he did it.”
Even Mr. Trump’s most loyal allies in the Senate had concerns.
Senator Lindsey Graham of South Carolina said that Mr. Trump had a right to his endorsement. But, he added, “you don’t have to be a rocket scientist to figure out the pathway for Paxton is there, but it’s more uphill.”
Democrats have been optimistic about their changes in Texas after nominating James Talarico, a state legislator who has focused on outreach to independent voters and boasts strong fund-raising numbers.
Mr. Graham said that he believed the Texas Senate race would likely become more expensive and competitive with Mr. Paxton as the nominee.
“What we’ve got to do is raise a lot more money now,” Mr. Graham said.
