Boston ? The fight for gay marriage appeared to be gaining ground a year ago. Although dozens of states had passed laws defining marriage as a heterosexual institution, advocates took heart in state-sanctioned civil unions in Vermont, expanded domestic partnership benefits in California, and a Supreme Court decision striking down the Texas sodomy ban.

Then, in a ruling hailed by supporters as the start of a new era, the highest court in Massachusetts made the state the first to sanction same-sex marriages.

As supporters celebrate the first anniversary of that ruling Wednesday, both sides are digging in for tougher, longer battles.

Opponents of gay marriage are bolstered by this year’s elections, when 11 states pushed through constitutional amendments banning gay marriage, joining six others that had done so earlier. And President Bush has promised to make a federal anti-gay marriage amendment a priority of his second term.

“I think what we’re seeing now is a visible manifestation of the momentum that has been building and will continue to build,” said Mat Staver, president of the Liberty Counsel, a conservative, Orlando, Fla.-based law group, which is involved in 30 cases around the nation.

But advocates aren’t giving in.

“Backlash simply means that you’re making forward progress,” said Josh Friedes, spokesman for the Massachusetts Freedom to Marry Coalition. “I think that people who say it suddenly seems like the radical right has momentum on their side, they’re not looking at the broad brushstroke of history.”

In April 2001, seven gay and lesbian couples who were denied marriage licenses in Massachusetts filed suit challenging the state’s gay marriage ban. Two and a half years later, a deeply divided Massachusetts Supreme Judicial Court found there was “no rational reason” for such a ban under the state’s constitution and ordered the state to start allowing gays to marry six months later.

In reaction, officials issued marriage licenses to gay couples in San Francisco and New York state, conservative and religious groups spent millions of dollars to support constitutional bans, and politicians called for removal of “activist judges.”

Earlier this year, Massachusetts lawmakers gave first-round approval to placing a proposed constitutional amendment on the November 2006 ballot that would ban gay marriages but allow Vermont-style civil unions.

Julie Goodridge, left, and her spouse, Hillary Goodridge, cross the street after being married May 17. The Goodridges were the lead plaintiffs in the Massachusetts gay marriage lawsuit. Supporters will celebrate the first anniversary of the landmark Massachusetts ruling Nov. 18.

Staver said he could feel passions rising as he spoke to people around the country.

“Even though there was a lot of lawsuits, my feeling was the same-sex marriage movement had moved too fast and it was going to be their Achilles’ heel,” he said.

Within a week of the Nov. 2 election, lawsuits were filed against the new gay marriage bans in Oklahoma and Georgia. Earlier, Louisiana’s ban was struck down by a state court that found it improperly dealt with more than one subject by banning same-sex marriage and any legal recognition of common-law relationships, domestic partnerships and civil unions.

Eight of the 11 state bans passed on Election Day prohibit civil unions as well as marriage.