Thomson Reuters
Senator Jon Tester (D-MT) speaks to reporters on Capitol Hill in Washington.
Gorsuch was "a smart man," Tester said in a statement, "but that doesn't make him right for a lifetime appointment to the US Supreme Court."
"With Judge Gorsuch on the bench, 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 citizens," Tester continued.
"These are not Montana values, which is why I cannot support this nomination."
Tester's statement didn't reference the FBI's investigation into the Trump campaign's ties to Russia, a talking point some Democrats have used to justify blocking the Gorsuch confirmation. It also made no mention of Merrick Garland, President Barack Obama's Supreme Court nominee whom Republican leaders did not grant a hearing.
Democrats have vowed to filibuster Gorsuch's confirmation, and Republicans would need 60 votes to move forward - meaning at least eight Democrats would have to vote with them.
Republicans have already scooped up three Democratic senators, who were facing pressure from constituents in their Republican-leaning states: West Virginia's Joe Manchin, North Dakota's Heidi Heitkamp, and Indiana's Joe Donnelly. But Tester's opposition makes it increasingly likely that the confirmation battle could feature a fundamental change in US
If the filibuster holds, Senate Majority Leader Mitch McConnell could employ the "nuclear option" - changing Senate rules to allow nominees to be confirmed with a simple majority instead of 60. 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" 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."