Two further Democratic Senators, both from conservative states, have announced their support for marriage equality in Facebook posts, leaving only four of their colleagues still opposed or unsure.

Joe Donnelly of Indiana posted on Facebook today: “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.”

Heidi Heitkamp of North Dakota said: “In 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. I view the ability of anyone to marry as a logical extension of this belief.

“The makeup of families is changing, but the importance of family is enduring,” she added.

They follow Florida Democrat Bill Nelson’s announcement yesterday that he now fully supports equal marriage.

Only four of the 53 Democratic Senators have yet to come out in support for marriage equality, compared to 43 out of 45 Republicans.

Earlier this week Republican Senator Mark Kirk declared his support for equal marriage, saying: “Life comes down to who you love and who loves you back – government has no place in the middle.”

Republican Senator Rob Portman announced that he  endorsed same-sex marriage on 15 March.

The announcement was particularly surprising as Mr Portman had been one of the original sponsors of DOMA. He said the coming out of his gay son had prompted him to rethink his views.

Democratic Senator Timothy Kaine also shifted his position on Tuesday to support equal marriage.

He followed Democratic Senators Mark Begich and Mark Warner, who both came out in support of same-sex marriagebefore the Supreme Court’s hearings on the subject, following the precedent set by Claire McCaskill. Senator McCaskill announced her support via a Tumblr post.