BOSTON (AP) – Fresh after the success of defending same-sex marriage from its latest attack, advocates say they have one more fight in Massachusetts: Repealing a 1913 law that bars same-sex couples in most other states from coming here to get married.

Some say the law – which says couples cannot be married here unless their unions would be legal in their home states – has its roots in the effort to block interracial marriage, and plan soon to strategize for its repeal.

Massachusetts is the only U.S. state where same-sex marriage is legal. A few states recognize civil unions for same-sex couples.

Opponents of same-sex marriage, including the former governor and now Republican presidential candidate Mitt Romney, have said repealing the law would make Massachusetts the “Las Vegas of gay marriage.”

“This radical social experiment will be exported to the other 49 states,” Kris Mineau, president of the Massachusetts Family Institute, said Friday.

A day earlier, lawmakers killed an effort to let voters decide on a constitutional amendment to ban same-sex marriage. The defeat for same-sex marriage opponents – after more than three years of debate – makes the successful mounting of a future challenge even more unlikely.

Now attention turns to the state’s 1913 law, which, if repealed, would mean same-sex couples from other states could legally marry in Massachusetts.

The possible impact of the marriages in other states is unknown. Most states prohibit same-sex marriage, but a Massachusetts certificate could provide the foundation for legal challenges. A court challenge was the basis of Massachusetts legalizing same-sex marriage, after same-sex couples were denied marriage licenses.

Opponents of the 1913 law say it was originally approved as part of a deal with states that barred interracial marriages and did not want couples fleeing to Massachusetts to marry. Others say there is little evidence to support that claim.

After same-sex marriage became legal in May 2004, hundreds of couples from other states came to Massachusetts to wed. But then-Gov. Romney directed municipal clerks not to give licenses to out-of-state couples, citing the 1913 law.

So far, only Rhode Island allows its same-sex couples to wed in Massachusetts. More than 170 marriages of same-sex couples from New York who wed in Massachusetts before July 2006 have also been deemed valid, because New York had not explicitly banned same-sex marriages until then.

The courts might not be as helpful to those who want to strike down the 1913 law.

The Supreme Judicial Court, which legalized same-sex marriage, upheld the 1913 law last year, ruling that Massachusetts “has a significant interest in not meddling in matters in which another state, the one where a couple actually resides, has a paramount interest.”

Mineau seconded that sentiment Friday.

“It will open the floodgates for Massachusetts to become the Mecca for same-sex marriage,” he said. “Their goal is to strike down the marriage restrictions in every state. Their launching pad will be Massachusetts.”

Arline Isaacson, co-chair of the Massachusetts Gay and Lesbian Political Caucus, said “no one is rushing” to push for the repeal but she is confident it will happen.

“In the short term, we want everyone to rest, breathe and appreciate the incredible victory that took place,” she said.

Marc Solomon, campaign director of MassEquality, said he expects to set up meetings with legislative leaders and the governor sometime soon to discuss moving a bill to repeal the law.

The state’s top three political leaders – Gov. Deval Patrick, House Speaker Salvatore DiMasi and Senate President Therese Murray – are all strong supporters of same-sex marriage who indicted they would support repeal of the law.