(CNN) - Sen. Tim Johnson, D-South Dakota, on Monday joined the wave of senators from his party who are endorsing same-sex marriage for the first time.
The third-term senator, who's not seeking re-election next year, said in a statement his "views have evolved sufficiently to support marriage equality legislation."
"This position doesn't require any religious denomination to alter any of its tenets; it simply forbids government from discrimination regarding who can marry whom," he said.
With his announcement, that leaves three Senate Democrats who have yet to come out in support for gay and lesbian couples to have the legal right to marry. All three come from conservative-leaning states. They include Sen. Mary Landrieu of Louisiana, Sen. Mark Pryor of Arkansas, and Sen. Joe Manchin of West Virginia.
Both Pryor and Landrieu are up for re-election next year, while Manchin's term ends in 2018.
Landrieu told CNN National Political Correspondent Jim Acosta in an interview Friday that she personally believes "people should love who they love and marry who they want to marry," but that her obligation rests with the people of Louisiana who elected her.
"My state has a very strong constitutional amendment not only against gay marriage but against gay partnerships. So I'm looking at the people of Louisiana trying to represent their interests," she said.
Two Republicans have announced support for same-sex marriage: Rob Portman of Ohio and Mark Kirk of Illinois. Meanwhile, Republican Sen. Lisa Murkowski recently said her views on the issue are "evolving."
The Supreme Court heard oral arguments last month in two cases on the issue, including a challenge to the federal Defense of Marriage Act that defines marriage as between a man and a woman. During that week, it was reported that Johnson reversed his position on DOMA, but he did not come out in full support of same-sex marriage at the time.
- CNN's Steve Brusk, Ashley Killough and Kevin Liptak contributed to this report.