Claire McCaskill, a Democrat representing the red state of Missouri, announced today that she intends to join the more liberal members of the Senate in filibustering Neil Gorsuch’s nomination to fill the Supreme Court seat that has been vacant since Justice Antonin Scalia’s death.

She cites a “rigid ideology that always puts the little guy under the boot of corporations.”

Her announcement brings the number of “no” votes on cloture for Gorsuch up to 39, meaning it would require just two more opponents to block him, unless Republicans move to abolish the filibuster as it pertains to Supreme Court nominees. McCaskill is up for reelection in 2018 and Donald Trump carried her home state by 19 points last fall, so she is one of the Democrats who appeared most likely to defect to his side, on cloture at least.

Sens. Joe Manchin (D-WV) and Heidi Heitkamp (D-ND), who are even more imperiled in 2018, have both already committed to voting for Gorsuch.

Jon Tester of Montana and Joe Donnelly of Indiana also both represent strongly Trumpy states and are as yet undecided. But McCaskill’s decision to filibuster will greatly increase the pressure on blue-state senators such as Mark Warner (D-VA), Michael Bennet (D-CO), Chris Coons (D-DE), and Pat Leahy (D-VT) to join Gorsuch’s opponents.

If Democrats do filibuster the nomination, it seems very likely that Republicans will respond by changing Senate rules to ban filibustering of Supreme Court nominations. When Democrats last held the Senate, they eventually responded to persistent GOP filibustering of Obama administration nominees by eliminating filibusters for executive branch jobs and lower court appointments. The seat became vacant after the 2014 midterms, by which time Republicans held an outright majority in the Senate, so when Barack Obama appointed Merrick Garland to fill the vacancy, filibuster rules didn’t become relevant — the GOP majority simply declined to hold a vote.

Progressive activists have been urging Democratic senators to filibuster Gorsuch and the senators are largely complying, even while Senate Democrats privately fret that it might make more strategic sense to take a dive on this vote and mount strong opposition to a hypothetical future nominee.