WASHINGTON (AP) – Jumping into a volatile election-year debate on gay and lesbian weddings, President Bush backed a constitutional amendment banning marriage for gays and lesbians – a move he said was needed to stop judges from changing the definition of the “most enduring human institution.”

“After more than two centuries of American jurisprudence and millennia of human experience, a few judges and local authorities are presuming to change the most fundamental institution of civilization,” the president said in urging Congress to approve such an amendment. “Their action has created confusion on an issue that requires clarity.”

Marriage cannot be severed from its “cultural, religious and natural” roots, Bush said.

Bush, who has cast himself as a “compassionate conservative,” left the door open for civil unions as an alternative to marriage for gays and lesbians.

He noted actions in Massachusetts where four judges on the highest court have indicated they will order the issuance of marriage licenses to applicants of the same gender in May of this year. In San Francisco, city officials have issued thousands of marriage licenses to people of the same sex. This, Bush said, is contrary to state law. A county in New Mexico also has issued marriage licenses to gay and lesbian couples, Bush said.

“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 uncertainty,” Bush said.

The conservative wing of his party has been anxious for Bush to follow up his rhetoric on the issue with action. In recent weeks, Bush has repeatedly said he was “troubled” by the Massachusetts court decision and the marriages of gay and lesbian couples in San Francisco, but stopped short of endorsing a constitutional amendment.

The Massachusetts Supreme Judicial Court recently ruled that it is unconstitutional to bar gay and lesbian couples from marriage. Gay and lesbian couples from Europe and more than 20 states have flocked to San Francisco City Hall since city officials decided to begin marrying gay and lesbian couples. More than 3,200 people already had taken vows to be “spouses for life.”

At least 38 states and the federal government have approved laws or amendments barring the recognition of marriage for gays and lesbians; the Utah House gave final legislative approval to a measure outlawing marriage for gays and lesbians and sent it to the governor, who has not taken a position on the bill.

White House press secretary Scott McClellan said Bush believes that legislation for such an amendment, submitted by Rep. Marilyn Musgrave, R-Colo., meets his principles in protecting the “sanctity of marriage” between men and women. But Bush did not specifically embrace any particular piece of legislation in his announcement. White House officials have said that support for Musgrave’s proposed amendment has been unraveling in the Senate.

The amendment that Musgrave and other lawmakers are backing in the House says that marriage “shall consist only of the union of a man and a woman.”

Bush’s comment that the states should be left free to define “legal arrangements other than marriage” indicates the president does not favor using a constitutional amendment to enact a federal ban on civil union or domestic partnership laws.

The proposed amendment backed by Musgrave and others in Congress is consistent with that, but some conservatives favor going further.

Recent polling suggests Bush is on solid political ground.

A nationwide CNN poll found that by a margin of 64-32, those surveyed said marriage for gays and lesbians should not be recognized in law as valid, with the same rights as traditional marriages.

On a separate question, 48 percent of those surveyed said it should be up to the federal government to pass laws regarding marriage for gays and lesbians, while another 46 percent said the states should take that role.

Sen. John Kerry, Bush’s likely Democratic opponent in this year’s election, says he opposes marriage for gays and lesbians. But he also opposes a federal constitutional amendment to ban them, because he says it is an issue for the states to decide, spokeswoman Stephanie Cutter said.

Kerry says he prefers civil unions and rejects any federal or state legislation that could be used to eliminate equal protections for gays and lesbians or other forms of recognition like civil unions.

Wide-ranging reaction reflected the controversial nature of the issue.

A major gay and lesbian Republican group, the Log Cabin Republicans, accused Bush of “pandering to the radical right” and “writing discrimination into the Constitution.” Patrick Guerriero, executive director of the group, said, “The president has certainly jeopardized over 1 million gay and lesbian Americans self-identified in exit polls who voted for him in the year 2000.”

The Democratic National Committee said the decision was purely political. “It is wrong to write discrimination into the U.S. Constitution and it is shameful to use attacks against gay and lesbian families as an election strategy,” DNC Chairman Terence McAuliffe said.

The American Center for Law and Justice, which focuses on family and religious issues, applauded Bush’s announcement, saying it “serves as a critical catalyst to energize and organize those who will work diligently to ensure that marriage remains an institution between on man and one woman.”