(CNN) - The ranks of Senate Democrats who back same-sex marriage grew Friday morning by two as Indiana's Joe Donnelly and North Dakota's Heidi Heitkamp came out publically in support of overturning a federal law banning marriages between same sex couples.
"With the recent Supreme Court arguments and accompanying public discussion of same-sex marriage, I have been thinking about my past positions and votes. In doing so, I have concluded that the right thing to do is to support marriage equality for all," Donnelly, elected last November, wrote in a statement on his Facebook page.
Heitkamp, who also won election in 2012, wrote that after "speaking with North Dakotans from every corner of our great state, and much personal reflection, 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."
Both Indiana and North Dakota are conservative-leaning states.
With Donnelly and Heitkamp's announcements, there are now four Senate Democrats who have not backed marriage rights for gay and lesbian couples: Tim Johnson of South Dakota, Joe Manchin of West Virginia, Mark Pryor of Arkansas and Mary Landrieu of Louisiana.
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 Republican senators have announced support for same-sex marriage: Rob Portman of Ohio and Mark Kirk of Illinois.
The Supreme Court heard oral arguments last week 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.