Florida Sen. Bill Nelson now supports gay marriage.
In a special column for the Tampa Bay Times, the moderate Democrat explained Thursday, "The civil rights and responsibilities for one must pertain to all. Thus, to discriminate against one class and not another is wrong for me."
Nelson pledged to add his name to the petition of senators asking the U.S. Supreme Court to declare the law that prohibits gay marriage unconstitutional.
Nelson had previously supported civil unions for gay people but had believed marriage should be between a man and a woman.
With Nelson's reversal, only six Senate Democrats remain opposed to gay marriage. A handful of Republicans, including Ohio Sen. Rob Portman, have also changed their stance.
Nelson's spokesman Dan McLaughlin told the Times that the senator came to his decision after many conversations with constituents, clergy and others.
"People say, 'Why now?'" McLaughlin said. "Well it's because the cases that went before the Supreme Court and the public debate that has ensued. During this discourse, Sen. Nelson has heard from a lot of constituents. The way he goes about his job is to always treat people with civility and respect and hear what they have to say."
McLaughlin said that Nelson was not influenced by other senators who have changed their mind.
A national poll released Thursday by Quinnipiac University showed American voters favor same-sex marriage 50-41 percent, compared to 47-43 percent in early March. In July 2008, voters opposed same-sex unions 55-36 percent.