3rd Senate Democrat pledges to support Trump's Supreme Court nominee

However, he's against the so-called nuclear option.

"I was deeply disappointed by the way the most recent Supreme Court nominee, Judge Garland, was treated by the Senate, but as Senator, I can only vote on the nominee that comes to the Senate floor," Donnelley said.

"However, I believe that we should keep the current 60-vote threshold for Supreme Court nominees," he added.

But according to Senate Majority Leader Mitch McConnell, who on Sunday refused to rule out the nuclear option, Gorsuch will be confirmed, on way or another, by Friday.

"Judge Gorsuch is going to be confirmed. The way in which that occurs is in the hands of the Democratic minority," McConnell told Fox News' Chris Wallace.

"I don't think we know" whether additional Democrats will sign on to avoid a filibuster -- a move Democratic Leader Chuck Schumer of New York vowed to carry out last month, McConnell added.

It was Democrats who invoked the nuclear option in 2013, then to allow confirmation of lower court judges.

Schumer has since said he regrets the decision, which he says he argued against four years ago. McConnell, who spoke out against the nuclear option in 2013, now says that "going nuclear" prevents inappropriate filibustering by the minority party.

ABC News' Ali Rogin contributed to this report.