Senator Mark Kirk announced on Tuesday that he supports gay marriage, suggesting a brush with death had helped shift his attitude as he became the second Republican to join dozens of Democratic senators who back homosexuals' right to wed.
"When I climbed the Capitol steps in January, I promised myself that I would return to the Senate with an open mind and greater respect for others," said Kirk, an Illinois senator who returned to the Senate in January almost a year after suffering a major stroke.
"Same-sex couples should have the right to civil marriage," he said in a statement. "Our time on this Earth is limited, I know that better than most. Life comes down to who you love and who loves you back - government has no place in the middle."
Public opinion polls show a majority of Americans believe that homosexuals should have the right to wed, and a growing number of politicians are declaring themselves in favor of same-sex marriage rights.
The Supreme Court heard arguments in two major cases on marriage equality last week.
But Republicans, who are generally more socially conservative than Democrats, remain largely opposed.
Ohio Senator Rob Portman became one of the most prominent members of the party to back gay marriage rights when he announced his support in mid-March, two years after his son told him he was gay.
Kirk, who holds the Senate seat once occupied by President Barack Obama, represents a Democratic-leaning state and is considered a fairly moderate Republican. Illinois' other senator, Democrat Richard Durbin, also supports gay marriage, as do all but seven of the 53 Democrats in the Senate.
Eight Democratic senators have come out in support of gay marriage within the last week.
Delaware Senator Tom Carper became the eighth on Tuesday with an announcement on his Facebook page. Pennsylvania's Bob Casey had become the seventh on Monday.
Kirk's support means that half of the 100-member U.S. Senate support gay marriage - 46 Democrats, two independents and two Republicans - as does Vice President Joe Biden, who would cast a deciding vote if the issue came up in the chamber.