Senator Carper made the announcement Tuesday on his Facebook page, saying he came to his decision "after a great deal of soul searching."
Carper invoked the Golden Rule while explaining his conclusion that all Americans should be able to marry the person they love, regardless of sexual orientation.
Carper had been one of a dwindling number of Democratic senators who opposed same-sex marriage, and liberal groups have been pressuring those lawmakers to change their minds.
Congressman John Carney had been holding out as well but came out in support of same-sex marriage Monday.
Senator Carper was unavailable for comment. His staff tells WDEL he's traveling in Michigan.
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