Conservative religious and political leaders rallied Sunday in opposition of gay marriage, arguing that their rights to religious expression are being threatened.

The event, being broadcast to churches nationally, is part of a larger effort to energize conservative voters before the Nov. 7 congressional elections.

Gov. Mitt Romney, a likely Republican candidate for president in 2008, joined several members of the Massachusetts clergy and an estimated 1,000 supporters later Sunday at the Tremont Temple Baptist Church to denounce gay marriage.

The Washington-based Family Research Council chose Boston for the site of its annual "Liberty Sunday" because Massachusetts is the only state that has legalized gay marriage.

"When we look at what has happened with same-sex marriage, as it began in this state and threatens to spread across the country, we've seen in its wake the loss of religious freedoms and the ability to speak out based upon one's moral convictions," Tony Perkins, the organization's president, said Sunday.

Eight states will vote in November on amendments banning gay marriage, following 20 states that previously approved bans.

Prompted by a ruling from its highest court, Massachusetts legalized gay marriage in 2004. Massachusetts lawmakers are expected to consider a proposed constitutional amendment banning same-sex marriage on Nov. 9.

David Parker, a speaker at the Boston event, was arrested last year after he refused to leave the grounds of his 6-year-old son's school in Lexington after officials said they would not remove the boy from discussions about homosexuality.

"When religious liberty is compromised, all liberty is compromised," Parker said at a news conference before the rally.

But the issue is less about liberty and more about political posturing, according to the Rev. Barry Lynn, the executive director of Americans United for Separation of Church and State.

Lynn said he's not worried that the Sunday night program will change the minds of voters. In a way, he said, they're preaching to the choir.

"But it's the choir that has become the majority in these elections ... (The Family Research Council) has escalated their rhetoric and are trying to use this as a fire under their supporters," making sure they cast their ballots in November.

Among the ballot initiatives in November to prohibit gay marriage, passage is considered certain in Idaho, South Carolina, South Dakota and Tennessee, but gay-rights strategists believe their side is at least competitive in Arizona, Colorado, Virginia and Wisconsin.