Another potential Democratic swing vote announced Friday that she will vote against Judge Neil Gorsuch, President Trump’s nominee for the U.S. Supreme Court.

Sen. Claire McCaskill (D-Mo.) admitted it was “a really difficult decision for me” but that she will vote against Gorsuch when his nomination comes up for a vote in the Senate next week — and will support a Democratic filibuster of him.

In an explanation posted on the essay site Medium, McCaskill acknowledged that she and 10 other Democrats have been facing intense political pressure to back Trump’s choice for the high court.

“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.

McCaskill, like other Democrats, said she is against Gorsuch because of his past rulings on the U.S. Court of Appeals for the 10th Circuit against a truck driver in an employment case; his refusal during his confirmation hearing to answer specific questions about Supreme Court precedent or potential future issues; and because, in McCaskill’s words, Gorsuch believes “that corporations have the same rights as people.”

Only two Democrats — Heidi Heitkamp (N.D.) and Joe Manchin III (W.Va.) — have said they plan to vote for Gorsuch. They and a handful of others are the subject of a $10 million television ad campaign backing Gorsuch and trying to pressure Democrats to support him. The Judicial Crisis Network, spending the bulk of the money, on Friday said it is spending $1 million in Montana and three other states: The group hopes to win over Sen. Jon Tester (D-Mont.).

Tester, McCaskill, Manchin, Heitkamp and Sen. Joe Donnelly (Ind.) are the most imperiled Democrats in next year’s election, each representing states that Trump won handily.

On Friday, Sens. Richard Blumenthal (D-Conn.) and Brian Schatz (D-Hawaii) also announced they are opposed to Gorsuch, meaning that at least 35 Democrats will vote against him and back a filibuster. Forty-one senators would be needed to sustain the filibuster and force Republicans to either withdraw Gorsuch or use their majority powers to change Senate procedure and allow him and future Supreme Court nominees to be confirmed by a simple majority vote.

Senate Minority Leader Charles E. Schumer (D-N.Y.) said in an interview with The Washington Post on Thursday that Republicans are facing “an uphill slog” in finding the support needed to confirm Gorsuch.

But Republicans have vowed that Gorsuch will be confirmed no matter what — signaling that they will change Senate procedure, if needed.