Al Gore has spoken in favour of marriage for gay and lesbian couples.
The former Vice President of the United States, who was narrowly and controversially defeated by George W Bush in the 2000 Presidential election, came out in favour of equal rights in a video clip posted on current TV, a cable and online station he co-founded.
“I think it’s wrong for the government to discriminate against people because of a person’s sexual orientation,” he said.
Mr Gore, who was awarded the 2007 Nobel Peace Prize for his work highlighting climate change, added that he could not understand why some people consider same-sex marriage to be a threat to the institution.
“Shouldn’t we be promoting the kind of faithfulness and loyalty to one partner regardless of sexual orientation?
“Because if we don’t do that, then to that extent, you are promoting promiscuity and promoting all the problems that can result from promiscuity.
“And the loyalty and love that people feel for one another when they fall in love ought to be celebrated and encouraged and shouldn’t be prevented by any form of discrimination in the law.”
None of the leading candidates for the Democratic nomination for President in 2008 support full gay marriage. Barack Obama, Hillary Clinton and John Edwards all back civil unions.
Same-sex marriage is legal in only one US state, Massachusetts.
Rhode Island and New Mexico are the only US states that have not explicitly either banned or allowed same-sex unions.
At the start of this year New Hampshire became the fourth US state to legalise civil unions.
In all nine states have gay and lesbian spousal rights in some form: Connecticut, Vermont, New Hampshire, New Jersey, Maine, California, Washington and Hawaii.
The state of Oregon was due to offer domestic partnerships from January 1st but a judge delayed the introduction of the law from taking effect until at least February pending legal challenges.