Republican Senator Lisa Murkowski has said she is “disturbed” by her party’s stance before President Donald Trump’s impeachment trial.
Mr Trump was this month impeached by the Democrat-run House for abuse of power and obstruction of Congress.
He now faces trial in the Republican-dominated Senate, whose members are supposed to remain impartial.
However, Senate Majority Leader Mitch McConnell has pledged “total co-ordination” with the White House.
Mr Trump, the third president in US history to be impeached, is unlikely to be removed from office because of the Republican control of the Senate.
The president has repeatedly described the impeachment proceedings as a “witch-hunt”.
Ms Murkowski told Alaska’s KTUU news channel that she was uncomfortable with Mr McConnell’s comments about “total co-ordination”.
“When I heard that I was disturbed,” she said.
The senator also said there should be distance between the White House and the Senate over how the trial is conducted. “To me it means that we have to take that step back from being hand in glove with the defence,” she said.
At the same time, she described the impeachment proceedings as “rushed”.
Ms Murkowski, a moderate Republican, has criticised President Trump on a number of policy issues. In October 2018, she opted not to vote to confirm Mr Trump’s nominee for the Supreme Court, after sexual assault allegations.