US Senator Bill Nelson endorsed gay marriage on Thursday, shifting the chamber in favor of rights for same-sex couples.

Nelson, a Democrat from Florida, became the 51st lawmaker in the 100-seat Senate to support gay unions, the latest in a string of lawmakers to change their position on the sensitive political issue.

In a statement to the Tampa Bay Times he quoted the Declaration of Independence, saying "if we are endowed by our creator with rights, then why shouldn't those be attainable by gays and lesbians?"

"Simply put, if the Lord made homosexuals as well as heterosexuals, why should I discriminate against their civil marriage?" he said. "I shouldn't, and I won't."

With Nelson reversing his position, there are now just six Senate Democrats still against marriage equality, mostly from more conservative states like Arkansas, North Dakota and West Virginia.

All 45 Republicans remained opposed until two weeks ago, when Rob Portman of Ohio became the first GOP senator to reverse his position and back gay marriage. Earlier this week Senator Mark Kirk of Illinois followed suit.

Nelson was the 11th senator in the past two weeks to announce his backing for equal marriage rights, according to the campaign group Freedom to Marry.

Nelson said he was adding his name to those petitioning the Supreme Court "to declare the law that prohibits gay marriage unconstitutional."

The political shift comes one week after the US Supreme Court heard arguments on the constitutionality of the Defense of Marriage Act.

That controversial 1996 law denies married gay and lesbian couples the same federal rights and benefits that heterosexual couples take for granted, from tax breaks and welfare benefits to access to a hospitalized spouse.