NEW YORK (AP) -- Worried that courts might open the door to gay marriage, a coalition of religious leaders and family-policy experts wants to amend the U.S. Constitution with a declaration that marriage can only be between a man and a woman.

The Federal Marriage Amendment, to be presented Thursday at a news conference in Washington, would require ratification by both houses of Congress and the legislatures of 38 states to become law.

Gay-rights advocates describe the initiative as "gay-bashing," while its backers say the amendment would prevent judges from setting family policies that lack public support.

Matt Daniels, executive director of the Alliance for Marriage, said coalition members decided to propose the amendment because of fears that Vermont's year-old civil union law--granting marriage-like rights to same-sex couples--would spawn lawsuits nationwide by gays seeking similar rights.

"Let's challenge the homosexual movement to play fair on the playing field of democracy," Daniels said. "If they want the benefits of marriage allocated to a wider circle of groups, they need to convince the majority of people that it's the right thing."

The idea of a marriage amendment was raised in 1998 by an alliance of conservative Christian groups. Daniels' coalition doesn't include those organizations, but its 45-member advisory board has a strong religious tilt, including leaders of major black denominations, an Episcopal bishop, several Roman Catholic officials, and representatives of Jewish and Muslim groups. Its lay members include professors from Princeton University, Amherst College and the law schools of Harvard, Notre Dame and Louisiana State.

Daniels said members of Congress from both parties were prepared to support the amendment, but their names will be released later--not at Thursday's news conference. "We believe this is more important than partisan politics, so we'll announce it with no politicians at the microphone," he said.

Thirty-four states already have adopted so-called "defense of marriage" laws, defining marriage as the union of a man and a woman. Daniels is optimistic that enough state legislatures would ratify the proposed amendment for it to pass. However, he said it might take several years to mobilize enough support in Congress. There have been 27 amendments to the Constitution--only one of them ratified since 1971.

"Congress won't act until the situation is more alarming to more people," Daniels said. "It will require more developments in the courts--but I have no doubt that it will happen."

Vermont legislators enacted the civil union bill last year in compliance with a state Supreme Court order saying same-sex couples were entitled to the same rights as heterosexual married couples. Since then, more than 2,300 civil union ceremonies have been performed in Vermont, mostly involving out-of-staters. Opponents of gay marriage predict that increasing numbers of those non-Vermont couples will turn to the courts in their own states, seeking recognition of their union.

One such case is now before Georgia's Court of Appeals, while seven Massachusetts couples have sued to overturn their state's ban on same-sex marriages.

The spokesman for a major gay-rights organization, David Smith of the Human Rights Council, denounced the proposed amendment as "a mean-spirited attack on gay families."

"It would create a constitutionally mandated second class of citizens," he said.

Daniels, however, said his Alexandria, Va.-based alliance was taking a "reasonable, centrist approach."

The amendment was supported in an editorial this month in the conservative magazine National Review, which called it "the only sure way to prevent a harmful and antidemocratic revolution in American law."

As drafted, the amendment would be just two sentences: "Marriage in the United States shall consist only of the union of a man and a woman.

"Neither this Constitution or the constitution of any state, nor state or federal law, shall be construed to require that marital status or the legal incidents thereof be conferred upon unmarried couples or groups."