Senator Bill Cassidy of Louisiana, one of seven Senate Republicans who voted to convict Donald J. Trump of inciting an insurrection in 2021, will depart Congress next year, after losing a Republican primary in which Mr. Trump lined up squarely against him.

Mr. Cassidy’s defeat means that at most two of those seven Republicans will remain in the Senate next year. One of them, Senator Susan Collins of Maine, faces a competitive general election campaign. The other, Senator Lisa Murkowski of Alaska, was re-elected in 2022.

Four of the seven opted for retirement over what might have been challenging re-election bids. Mr. Cassidy is the first to lose in a primary.

Mr. Trump’s 2021 impeachment trial resulted in an acquittal by the Senate. On Capitol Hill, the ranks of Republicans who are critical of Mr. Trump have been shrinking, reflecting his yearslong dominance in the party.

Here are the seven Senate Republicans who voted to convict Mr. Trump:

Richard M. Burr of North Carolina: Mr. Burr, 70, did not seek re-election in 2022. He was censured by the North Carolina Republican Party after his vote. He now works at a law firm.

Patrick J. Toomey of Pennsylvania: Mr. Toomey, 64, did not seek re-election in 2022. At the time of the 2021 vote, he accused Mr. Trump of betraying “the Constitution and his oath of office.” He now serves on the board of directors at Apollo Global Management, a private-equity firm.

Ben Sasse of Nebraska: Mr. Sasse, 54, left the Senate in 2023 to become the president of the University of Florida. (Mr. Trump hailed his exit, writing on social media that it was “great news!”) Mr. Sasse left the University of Florida in 2024, citing his wife’s deteriorating health. In December, he announced that he had been diagnosed with terminal Stage 4 pancreatic cancer.

Mitt Romney of Utah: Mr. Romney, 79, did not seek re-election in 2024. A frequent and unsparing critic of Mr. Trump, Mr. Romney was the only Senate Republican to vote to convict Mr. Trump in both of his impeachment trials. Mr. Romney said he was retiring to make room for a new generation of leaders. Last year, he told the student newspaper at Southern Methodist University, The Daily Campus, that he was enjoying speaking on college campuses and spending time with his family.

Senator Lisa Murkowski of Alaska: Ms. Murkowski, 68, won re-election in her independent-minded state in 2022, defeating a rival backed by Mr. Trump. She is the lone Republican senator to win re-election after voting to convict Mr. Trump in the 2021 trial.

Senator Susan Collins of Maine: Ms. Collins, 73, is running for re-election this year and is considered one of the most vulnerable Republicans in the country in the fall. Ms. Collins does not have a primary challenger and is in general-election mode. Democrats are pointing to areas where she has backed Mr. Trump. (She recently attended an event hosted by Mr. Romney in Utah, his spokeswoman, Liz Johnson, said.)

Senator Bill Cassidy of Louisiana: Mr. Cassidy, 68, finished third in the primary. He ended the race behind Representative Julia Letlow, who was backed by Mr. Trump, and State Treasurer John Fleming, a former congressman who has highlighted his work in the first Trump White House.