Sen. Jon Tester of Montana speaking with reporters on Capitol Hill. Thomson Reuters Democratic Sen. Jon Tester of Montana on Sunday said he would vote against President Donald Trump's nominee for the Supreme Court, complicating Republicans' efforts to confirm the judge without a fundamental Senate rule change.

In a statement, Tester called the nominee, Judge Neil Gorsuch, "a smart man" but said "that doesn't make him right for a lifetime appointment to the US Supreme Court."

"With Judge Gorsuch on the bench," Tester added, "I am deeply concerned that dark money will continue to drown out the voices and votes of citizens, the Court will stand between women and their doctors, and the government will reach into the private lives of law-abiding Americans.

"These are not Montana values, which is why I cannot support this nomination."

Tester's statement didn't mention the FBI's investigation into the Trump campaign's ties to Russia, a talking point some Democrats have used to justify trying to block Gorsuch's confirmation to the Supreme Court seat left vacant since the death of Justice Antonin Scalia. It also made no mention of Merrick Garland, President Barack Obama's Supreme Court nominee to fill the seat, whom Republican leaders did not grant a hearing.

Democrats have vowed to filibuster Gorsuch's confirmation. If they do, Republicans would need 60 votes to move forward, meaning at least eight Democrats would have to vote with them. Tester was viewed as a necessary pick-off for Republicans for Gorsuch to get past the 60-vote threshold.

Republicans have already scooped up three Democratic senators who were facing pressure from constituents in their Republican-leaning states: Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota, and Joe Donnelly of Indiana. But Tester's opposition makes it increasingly likely that the confirmation battle could feature a fundamental change in US politics.

If the filibuster holds, Senate Majority Leader Mitch McConnell could employ the "nuclear option" — changing Senate rules to allow Supreme Court nominees to be confirmed with a simple majority instead of 60 votes. McConnell has expressed reluctance to do so but hasn't ruled out the option.

"Neil Gorsuch will be confirmed this week," he said on "Meet the Press" on Sunday. "How that happens really depends on our Democratic friends, how many of them are willing to oppose cloture on a partisan basis to kill a Supreme Court nominee, never happened before in history, the whole history of the country."