"It calls on us to treat others as we want to be treated," Carper wrote. "That means, to me, that all Americans ultimately should be free to marry the people they love and intend to share their lives with, regardless of their sexual orientation, and that's why today, after a great deal of soul searching, I'm endorsing marriage equality."

The Supreme Court last week heard oral arguments on both a challenge of the Defense of Marriage Act (DOMA), which bars the government from distributing certain spousal benefits to same-sex couples, and a California ballot initiative that banned gay marriage in the state. Gay-rights activists hope a favorable ruling could strike down prohibitions on same-sex marriage across the United States.

Casey's decision means now just seven of the 53 Democrats in the Senate do not support gay marriage. But every Republican senator except Ohio's Rob PortmanRobert (Rob) Jones PortmanMcConnell names Senate GOP tax conferees Overnight Finance: House approves motion to go to tax conference — with drama | GOP leaders to consider Dec. 30 spending bill | Justices skeptical of ban on sports betting | Mulvaney won't fire official who sued him How four GOP senators guided a tax-bill victory behind the scenes MORE — who said last month that his gay son had led him to change his mind — opposes same-sex marriages.

Carper had previously said he believed that DOMA should be repealed, but said that same-sex marriages should be left up to the states.