Washington (CNN) Sen. Claire McCaskill will not vote for Supreme Court nominee Neil Gorsuch and will not vote for the procedural motion to advance his nomination, the Missouri Democrat announced Friday.

"While I have come to the conclusion that I can't support Neil Gorsuch for the Supreme Court -- and will vote no on the procedural vote and his confirmation -- I remain very worried about our polarized politics and what the future will bring, since I'm certain we will have a Senate rule change that will usher in more extreme judges in the future," McCaskill wrote in a post on Medium

Republicans hoped to persuade McCaskill and other Democratic senators to help break a filibuster to advance Gorsuch's nomination. The filibuster requires 60 votes to break and Republicans only hold 52 seats. Democratic Sens. Heidi Heitkamp and Joe Manchin are so far the only two Democratic senators to say they'll back Gorsuch.

McCaskill had seemed like a particularly open-minded possibility following a Kansas City Star report earlier this week, which featured comments she made a behind closed doors warning Democratic donors about the consequences to blocking Gorsuch's nomination.

"By the way, Gorsuch was one of the better ones," McCaskill reportedly said.

White House press secretary Sean Spicer highlighted her remarks during Friday's briefing.

"We hope that her praise leads to additional support and her support. It's hard to find any reason except for obstructionism to see why fellow Democrats in her caucus have not been able to join them," Spicer said.

When asked by reporters on the Hill Friday about her comments in the Star, McCaskill said "I think my words speak for themselves."

"I think it's really interesting that the Republicans wanted to out the fact that I was so honest about how torn I am," she added.

Without McCaskill, Republicans are mulling whether to change Senate rules to lower the threshold to just 51 votes to advance Supreme Court nominations, also known as the "nuclear option." Some Senate traditionalists have seen the nuclear option as damaging to the Senate's role, but many Republican senators have said they're willing to use it if Democrats block Gorsuch. Changing the rules only requires a majority of the senators present at the vote.