By Susan Heavey

WASHINGTON (Reuters) - Former Secretary of State Hillary Clinton, a potential 2016 presidential candidate, said on Monday she backs marriage rights for gay Americans.

In a video for the gay rights advocacy group Human Rights Campaign, Clinton said she supports gay marriage "personally and as a matter of policy and law."

Lesbian, gay, bisexual and transgender "Americans are our colleagues, our teachers, our soldiers, our friends, our loved ones, and they are full and equal citizens and deserve the rights of citizenship. That includes marriage," Clinton said in the 5-minute-long video posted on YouTube (http://youtu.be/6RP9pbKMJ7c).

Her comments come amid a wave of similar declarations from other politicians, both Democrats and Republicans, and as the Supreme Court is considering two gay marriage cases.

President Barack Obama last year announced his support for gay marriage, which is already permitted in nine of the 50 states and Washington, D.C.

Earlier this month, Clinton's husband, former President Bill Clinton, also addressed the issue, saying the 1996 law he signed defining marriage as between a man and woman was unconstitutional and should be overturned.

The Republican Party opposed gay marriage in its 2012 convention platform, but some notable Republicans have broken ranks.

Among them is Ohio Senator Rob Portman, who on Friday became the most prominent Republican lawmaker to support gay marriage, influenced, he said, by learning that his son was gay. Republican leaders, however, were quick to reiterate their opposition to gay marriage.

During her unsuccessful campaign for the 2008 Democratic presidential nomination, Clinton said she supported civil unions for gay couples but did not say she backed same-sex marriage.

Clinton has been coy about her political future after stepping down as secretary of state earlier this year, but polls show she maintains her popularity as a potential Democratic presidential nominee in the 2016 election.

Explaining how her view had changed, Clinton cited her human rights work during her four years as the top diplomat and her own faith. She also cited the happiness she and her husband felt when their daughter, Chelsea Clinton, married a few years ago.

"I wish every parent that same joy," she said.

(Reporting by Susan Heavey; Editing by Fred Barbash and Mohammad Zargham)