The U.S. Senate is headed for a tense showdown this week over President Donald Trump's first Supreme Court nominee that could have far-reaching consequences for Congress, the high court and the nation.

With 52 Republican senators on board for Neil Gorsuch's nomination, eight votes from Democrats or the Senate's two independents would be needed to advance the nomination and prevent a filibuster.

Two Democratic senators on Sunday split over supporting Judge Neil Gorsuch's nomination to the Supreme Court.

Joe Donnelly of Indiana said he would vote in favor of Gorsuch's confirmation while Jon Tester of Montana announced he would not back the federal appeals court judge based in Denver.

Donnelly became the third Democrat to break with the party as Republicans line up behind President Donald Trump's choice for the high court.

So far, only Donnelly of Indiana, Heidi Heitkamp of North Dakota and Joe Manchin of West Virginia — all representing states Trump won in November and all up for re-election next year — have said they will vote to confirm Gorsuch.

Republican Senator Lindsey Graham speaks at a rally for nominee Neil Gorsuch outside the U.S. Supreme Court in Washington, D.C. on March 29. (Aaron P. Bernstein/Reuters )

Tester represents a state won by Trump and faces re-election, too, but he said Gorsuch did not directly answer questions when the two met or during the confirmation hearing. Tester said he based his decision on the judge's past cases, noting that he found troubling Gorsuch's record on privacy and that he believes Gorsuch places corporations over people.

Donnelly called Gorsuch, 49, "a qualified jurist who will base his decisions on his understanding of the law and is well-respected among his peers."

Hours before Donnelly's and Tester's announcements, Senate Minority Leader Chuck Schumer of New York said he didn't expect Gorsuch to receive 60 votes to overcome a filibuster threat.

If Democrats mounted a filibuster, Senator Majority Leader Mitch McConnell of Kentucky was expected to seek a change in Senate rules allowing a simple majority of the 100-member Senate to confirm the nomination.

"Neil Gorsuch will be confirmed this week," McConnell said on CNN's State of the Union.

He added: "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."

Such a change in Senate rules — known as the "nuclear option" — would likely be retained in the future and thus make Supreme Court confirmations more susceptible to simple party-line votes instead of bipartisan support.