Sen. Claire McCaskill Claire Conner McCaskillDemocratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally Missouri county issues travel advisory for Lake of the Ozarks after Memorial Day parties MORE (D-Mo.) on Friday said she will vote to support a filibuster of President Trump’s Supreme Court nominee, Neil Gorsuch.

The announcement makes it significantly harder for Gorsuch to muster the 60 votes he needs to overcome a filibuster and advance to a final confirmation vote.

McCaskill is the first Democrat facing reelection next year in a state President Trump carried by double digits to come out against Gorsuch.

She announced her opposition in a statement posted to Medium, faulting the nominee for “a stunning lack of humanity.”

“I cannot support Judge Gorsuch because a study of his opinions reveal a rigid ideology that always puts the little guy under the boot of corporations,” she wrote.

McCaskill cited the judge's opinion in Transam Trucking v. Administrative Review Board, which sided with an employer who ordered a trucker to stay with a disabled vehicle in sub-zero weather conditions.

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She said the nomination of Gorsuch goes against the grain of Trump’s promise to help working-class Americans because he is “a judge who can’t even see them.”

McCaskill also raised concerns about Gorsuch’s refusal during his confirmation hearing to say how he viewed the constitutionality of campaign fundraising regulations, which were limited by the landmark case Citizens United v. Federal Election Commission in 2010.

“I cannot and will not support a nominee that allows dark and dirty anonymous money to continue to flood unchecked into our elections,” she said.

McCaskill acknowledged that it was “a really difficult decision for me.”

An audio recording of McCaskill’s comments to a group of Democratic donors that surfaced Thursday revealed she was worried blocking Gorsuch could backfire on the party.

She warned that if Republicans ax the filibuster for Supreme Court nominees, it would allow Trump to appoint an extreme conservative to fill a future vacancy.

McCaskill has previously expressed concern about the threat of a liberal primary challenger in 2018. Republicans on Friday accused her of pandering to liberal members of the base.

National Republican Senatorial Committee spokeswoman Katie Martin said McCaskill’s “decision today might appease the extreme liberal elite but, unfortunately for Claire, those folks don’t vote in Missouri.”

Her "no" vote shrinks the pool of Democrats who have undecided or unclear positions on Gorsuch to nine. Gorsuch's nomination needs the backing of eight Democrats or Independents, along with all 52 Republicans, to break a filibuster.

Only two Democrats have so far said they will vote to end a filibuster of Gorsuch and support his final confirmation, according to The Hill’s Whip List. Both of them, Sens. Joe Manchin Joseph (Joe) ManchinBiden promises Democratic senators help in battleground states Senate leaders quash talk of rank-and-file COVID-19 deal OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' MORE (W.Va.) and Heidi Heitkamp Mary (Heidi) Kathryn HeitkampCentrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents Susan Collins set to play pivotal role in impeachment drama MORE (N.D.), represent states Trump won overwhelmingly in November.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.) has vowed that Gorsuch will be confirmed and has told colleagues to expect a vote to change the rules to lower the threshold for ending a filibuster to a simple majority.

To avoid a showdown over the rules, it now becomes crucial for Gorsuch to pick up the support of the two remaining undecided Democrats who face reelection next year in strongly pro-Trump states: Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (Mont.) and Sen. Joe Donnelly Joseph (Joe) Simon DonnellyEx-Sen. Joe Donnelly endorses Biden Lobbying world 70 former senators propose bipartisan caucus for incumbents MORE (Ind.).

Gorsuch would likely also need the support of senior Democrats such as Sen. Dianne Feinstein Dianne Emiel FeinsteinMcConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Intensifying natural disasters do little to move needle on climate efforts MORE (Calif.), the ranking member on the Judiciary Committee, and Sen. Patrick Leahy Patrick Joseph LeahyBattle over timing complicates Democratic shutdown strategy Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (Vt.), who might be concerned about preserving their power to filibuster for the next vacancy on the court.

Other Democrats up in the air are centrist Sens. Mark Warner Mark Robert WarnerIntelligence chief says Congress will get some in-person election security briefings Overnight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' House approves bill to secure internet-connected federal devices against cyber threats MORE (Va.) and Chris Coons Christopher (Chris) Andrew CoonsBiden promises Democratic senators help in battleground states Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep MORE (Del.), along with Independent Sen. Angus King Angus KingShakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' On The Trail: How Nancy Pelosi could improbably become president Angus King: Ending election security briefings 'looks like a pre-cover-up' MORE (Maine), who praised Gorsuch earlier this year as “exceedingly independent.”

Updated: 5:13 p.m.