Sen. Joe Donnelly Joseph (Joe) Simon DonnellyHarris faces pivotal moment with Supreme Court battle Trump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court MORE (Ind.), one of only three Democrats who voted for Neil Gorsuch, President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE’s first Supreme Court nominee, says he will oppose Brett Kavanaugh, Trump’s second pick to the high court.

Donnelly, who faces a tough reelection in a state that Trump carried by double digits in 2016, was considered one of the Democrats most likely to back Kavanaugh, whose nomination has become consumed by allegations of sexual misconduct.

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“I have deep reservations about Judge Kavanaugh’s nomination to this lifetime position and, as I stated, we have been unable to get all the information necessary regarding this nomination, despite my best efforts. Only 113 people have ever served on the Supreme Court, and I believe that we must do our level best to protect its sanctity,” Donnelly said in a statement Friday morning.

Donnelly's Republican opponent, businessman Mike Braun, immediately attacked his decision, calling it "a grave mistake."

Braun declared the move “proves he is more concerned with standing with his liberal Democrat leaders than standing for Hoosiers."

"I continue to strongly support Judge Kavanaugh's confirmation to the Supreme Court," he added.

Braun accused Democrats of creating "a media circus designed to smear and destroy Judge Kavanaugh's reputation."

His announcement leaves only a few Democrats undecided: Sens. Joe Manchin Joseph (Joe) ManchinSenate passes resolution reaffirming commitment to peaceful transition of power Hopes for DC, Puerto Rico statehood rise Manchin defends Supreme Court candidate Barrett: 'It's awful to bring in religion' MORE (W.Va.), Jon Tester Jonathan (Jon) TesterDemocrats shoot down talk of expanding Supreme Court Pence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans MORE (Mont.) and Heidi Heitkamp Mary (Heidi) Kathryn HeitkampHarris faces pivotal moment with Supreme Court battle Centrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents MORE (N.D.), all of whom also face challenging reelection fights in states Trump easily carried.

Donnelly said he did not have enough information to assess Christine Blasey Ford’s allegation that Kavanaugh sexually assaulted her at a high school party, as she described in emotional testimony before the Senate Judiciary Committee on Thursday.

“My job as a Senator is to gather as much information as I can to make the best-informed decision. The allegations made against Judge Kavanaugh are disturbing and credible. In the interest of getting as much information as possible, I believe the allegations should be investigated by the FBI,” he said.

Donnelly coming out in opposition, despite representing a state that Trump won by nearly 20 points, will put pressure on other Democrats to do the same.

—Updated at 12:18 p.m.