The senator is one of 10 Democratic senators vulnerable in the 2018 elections.
Senator Joe Donnelly has become the third Democratic Senator to throw their support behind President Donald Trump's Supreme Court nomination.
Senator Donnelly joins Senator Heidi Heitkamp of North Dakota and Senator Joe Manchin of West Virginia in backing Neil Gorsuch.
"After meeting with Judge Gorsuch, conducting a thorough review of his record, and closely following his hearing before the Senate Judiciary Committee, I believe that he is a qualified jurist who will base his decisions on his understanding of the law and is well-respected among his peers," the Indiana Democrat said in a statement.
As it stands, 35 senators have said or suggested they will filibuster Gorsuch's nomination - an obstructive tactic used to prevent a measure from being brought to a vote by extending debate on the topic.
Another two Democrats said they'll oppose Gorsuch in the final confirmation vote, but it's unclear where they will stand on a potential filibuster.
Senator Donnelly added the Senate "should keep the current 60-vote threshold for Supreme Court nominees."
Senator Donnelly's move will prove to be a curveball for Senate Minority Leader Chuck Schumer, who told US media earlier that that he would be able to unite enough Democrats to deny Gorsuch the 60 votes the Republican leadership will need to overcome a filibuster.