"I have concluded the federal government should no longer discriminate against people who want to make lifelong, loving commitments to each other or interfere in personal, private, and intimate relationships," Heitkamp said. "I view the ability of anyone to marry as a logical extension of this belief."
Donnelly said he reconsidered his opposition to gay marriage after recent Supreme Court arguments and public discussion on the issue.
"I have been thinking about my past positions and votes," he said. "In doing so, I have concluded that the right thing to do is to support marriage equality for all."
Donnelly and Heitkamp are the latest Democratic senators to announce their support for gay marriage. On Thursday, Florida Sen. Bill Nelson became the 51st senator to announce support for gay marriage. Both Donnelly and Heitkamp were elected in 2012, winning close races in states that were handily carried by GOP Presidential nominee Mitt Romney.
There are now only four Democrats in the Senate who have not expressed support for gay marriage: Sens. Mark Pryor, of Arkansas, Mary Landrieu of Louisiana, Tim Johnson of South Dakota, and Joe Manchin, of West Virginia.
Two Republican Senators, Mark Kirk of Illinois and Rob Portman of Ohio, have announced their support for gay marriage.