Senate Majority Leader John Thune is not going to endorse one of the two Louisiana Republicans vying in a runoff to replace Sen. Bill Cassidy, who was ousted from the race Saturday.
Mr. Thune had endorsed Mr. Cassidy’s bid for a third term, but President Trump opposed the incumbent returning to the Senate and backed Rep. Julia Letlow instead.
Ms. Letlow placed first in Saturday’s primary with 45% of the vote and advanced to the June 27 runoff against state Treasurer John Fleming, who placed second with 28%.
Mr. Cassidy placed third with 25% and was knocked out of the running.
Mr. Thune, South Dakota Republican, told reporters Monday that he is “not weighing in on” the runoff between Ms. Letlow and Mr. Fleming.
The winner is effectively guaranteed to be the next senator since Louisiana is a conservative state and Democrats did not put up a significant challenger.
The president’s opposition to Mr. Cassidy stems from a five-year-old grudge. The senator voted to convict Mr. Trump in his 2021 impeachment trial over charges “incitement of insurrection” at the Capitol on Jan. 6.
Mr. Thune acknowledged the “challenges” the incumbent faced because of that vote and not receiving the president’s endorsement but nonetheless praised Mr. Cassidy as “a very principled conservative, a very independent thinker.”
“The Republican voters of Louisiana have spoken, so we’ll move on, but he’s got several months here in which he can be a real force for change and a factor in trying to get some things done,” Mr. Thune said.
“He chairs an incredibly significant, powerful committee here, and we look forward to continuing to work with him,” the majority leader said, referring to his post on the Senate Health, Education, Labor and Pensions Committee.
Mr. Thune said he does not envision Mr. Cassidy using his loss as a reason to obstruct the president’s legislative agenda.
“I think Cassidy is a team player, and I think he obviously wants to see our team succeed,” he said.

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