The top Democrat in the U.S. Senate said on Sunday it was unlikely that Supreme Court nominee Neil Gorsuch would be able to clear a procedural hurdle to a final vote, even as a third Senate Democrat threw support behind the pick.

Senator Joe Donnelly, a Democrat from Indiana, said he would vote in favor of Gorsuch, calling him in a statement a "qualified jurist who ... is well-respected among his peers."

Donnelly joined Senators Joe Manchin of West Virginia and Heidi Heitkamp of North Dakota as the only Senate Democrats to announce support for President Donald Trump's court pick.

If confirmed to fill a vacancy created by the February 2016 death of conservative Justice Antonin Scalia, Gorsuch, 49, an appeals court judge, would restore the nine-seat high court's conservative majority, fulfilling one of Trump's top campaign promises.

Most other Democrats have made their opposition clear and 37 senators have indicated support for a filibuster of the nomination, which would force Republicans to come up with 60 votes to move forward. Republicans control the Senate 52-48.

"It's highly, highly unlikely that he'll get to 60," Senate Democratic leader Chuck Schumer said on NBC's "Meet the Press."

On Sunday evening, Democratic Senator Jon Tester declared he would vote against Gorsuch. In a statement, the Montana lawmaker said Gorsuch was "smart" but added: "That doesn't make him right for a lifetime appointment to the U.S. Supreme Court."

If the planned filibuster holds, Senate Republican leader Mitch McConnell would only be able to advance the nomination of Gorsuch by changing long-standing Senate rules so it could be approved by a simple majority.

McConnell, who also appeared on "Meet the Press," declined to rule out that option, which the Republican president has urged, and vowed the Senate would confirm Gorsuch one way or another.

"What I can tell you is Neil Gorsuch will be confirmed this week. How that happens really depends on our Democratic friends," he said.

The Senate Judiciary Committee is set to vote on the nomination on Monday, setting the stage for what McConnell has said will be a vote in the full Senate on Friday.

McConnell declined to say if he had sufficient support among his fellow Republicans to change the Senate's rules.