WASHINGTON - President Bush called yesterday for a constitutional amendment that would ban gay marriage, pushing an explosive cultural issue to the forefront of the 2004 presidential campaign.

In a brief announcement, Bush urged Congress to pass an amendment to the Constitution "defining and protecting marriage as a union of a man and a woman as husband and wife."

The move followed actions in Massachusetts and San Francisco, where the mayor recently began issuing marriage licenses to gay and lesbian couples. Massachusetts' highest court ruled in November that same-sex couples have the same right as heterosexuals to marry and ordered the state to begin issuing marriage licenses to them in May.

"Unless action is taken, we can expect more arbitrary court decisions, more litigation, more defiance of the law by local officials, all of which adds to the uncertainty," said Bush, who alleged yesterday that San Francisco was violating state law.

To the delight of conservatives who have pressed Bush to speak out for such an amendment, the president said he had decided that action is needed because "a few judges and local authorities are presuming to change the most fundamental institution of civilization," and that such changes "could have serious consequences throughout the country."

Bush said an amendment could leave states free to allow other sorts of "legal arrangements other than marriage," such as the civil unions recognized in California and Vermont.

"The union of a man and a woman is the most enduring human institution," he said.

He declared that the bond between a husband and wife "promotes the welfare of children and the stability of society. Marriage cannot be severed from its cultural, religious and natural roots, without weakening the good influence of society."

Bush's endorsement was met with anger and outrage by many Democrats and civil liberties and gay-rights groups.

"Not since the days of Jim Crow segregation has our nation faced the prospect of discrimination written into law in such a shameful way," said David Tseng, executive director of Parents, Families and Friends of Lesbians and Gays. "Millions of Americans are disappointed that their president ... has bowed to political pressure to support the codification of hatred into our beloved Constitution."

Christopher E. Anders, legislative counsel for the American Civil Liberties Union, called the proposal "the nuclear bomb of anti-gay attacks" and said it could undermine state domestic partnership, adoption, foster care and kinship care laws.

The two leading Democratic presidential hopefuls, Sens. John Kerry of Massachusetts and John Edwards of North Carolina, oppose gay marriage but also oppose a constitutional amendment to ban it. They accused Bush of trying to divert attention from areas where he is vulnerable, such as the economy or Iraq. The White House denied the assertion.

Kerry said, "All Americans should be concerned when a president who is in political trouble tries to tamper with the Constitution of the United States at the start of his re-election campaign."

Edwards said: "We have had our Constitution for more than 200 years. We amended it to abolish slavery and ensure women could vote. We should not amend it over politics."

The prospects for passage of a constitutional amendment are far from clear, and the hurdles are steep. Two-thirds of each chamber of Congress and three-fourths of the states - 38 - would have to approve the proposal.

It is not clear whether Republicans have the support on Capitol Hill to see such a measure through, especially in an election year.

But conservatives, who are mounting one of the most serious efforts to amend the Constitution in the past quarter-century, said they were delighted to have presidential support.

"I've been urging him to do this for some time," said Paul Weyrich, head of the Free Congress Foundation. "Right now, I'd like to pay the mayor of San Francisco to keep up what he's doing. That has helped galvanize people."

In the past two weeks, since San Francisco officials started issuing marriage licenses to same-sex couples, more than 3,200 couples from around the country, and Europe, have flocked to the city to marry. A county in New Mexico has also started licensing gay marriages.