Just three Republicans — Senators Lindsey Graham of South Carolina, Susan Collins of Maine and Lisa Murkowski of Alaska — voted in June to confirm Judge Ketanji Brown Jackson, who is considered a front-runner to succeed Justice Stephen G. Breyer, to the U.S. Court of Appeals for the District of Columbia Circuit.

“I think she’s qualified for the job,” Mr. Graham, a senior member of the Judiciary Committee, told reporters at the time. “She has a different philosophy than I do.”

But backing someone for an appeals court post does not guarantee the same level of support for a high court vacancy. Multiple senators have voted against Supreme Court nominees they had previously backed.

Ms. Murkowski, a centrist Republican who is seeking re-election this year, has previously gone her own way on high court nominations, opposing President Donald J. Trump’s choice of Justice Brett M. Kavanaugh in 2018 but backing his nomination of Justice Amy Coney Barrett in 2020. Ms. Collins, another closely watched senator on Supreme Court nominations, voted to confirm Mr. Kavanaugh but opposed Ms. Barrett.