U.S. Sen. Shelley Moore Capito was one of 12 Republicans to vote to advance a federal law protecting same-sex and interracial marriages.
The Wednesday Senate vote of 62-37 means the Respect For Marriage Act is likely to see final passage.
The legislation codifies the U.S. Supreme Court decisions in recent years protecting the marriage rights of same-sex couples. It also protects the 1967 decision that struck down state laws banning interracial marriage.
The bill repeals the 1996 Defense of Marriage Act, which defined marriage as between one man and one woman for federal purposes.
Democrat Joe Manchin also voted to advance the bill.
Also Wednesday, Capito was elected by her Senate Republican colleagues to a leadership position. She is now vice-chair of the Senate Republican conference, the No. 4 post.
She is the first West Virginian to serve in Senate leadership since Robert Byrd in the 1980s.