"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 in the White House’s Roosevelt Room. It was a statement that was sure to please his conservative backers.

Bush, who has cast himself as a "compassionate conservative," left the door open for civil unions as an alternative to same-sex marriages.

His position was quickly denounced by liberals.

Democrats accused Bush of tinkering with a document that is the bedrock of American democracy to divert election-year attention away from his record — an allegation the White House denied. Sen. John Kerry, D-Mass., who hopes to run against Bush in this year’s presidential election, said: "I believe President Bush is wrong."

"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," said Kerry, who opposes gay marriage and will oppose the amendment if it reaches the Senate floor. Bush is "looking for a wedge issue to divide the American people," Kerry said.

Anthony Romero, director of the American Civil Liberties Union, said amending the Constitution to deny homosexuals the same rights that others take for granted "just isn’t very American." A major gay Republican group, the Log Cabin Republicans, accused Bush of "pandering to the radical right" and "writing discrimination into the Constitution."

Bush 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 same-sex marriage licenses, 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.

However, California Republican Reps. David Dreier and Jerry Lewis said a constitutional amendment might not be necessary.

"I will say that I’m not supportive of amending the Constitution on this issue," said Dreier, a co-chairman of Bush’s campaign in California in 2000. "I believe that this should go through the courts, and I think that we’re at a point where it’s not necessary."

Lewis said, "At this moment I feel changing the Constitution should be a last resort on almost any issue."

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 recent rulings on the issue, but stopped short of endorsing a constitutional amendment.

The Massachusetts Supreme Judicial Court recently ruled that it is unconstitutional to bar gay 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 same-sex couples. More than 3,200 couples had taken vows by Monday evening, promising to be "spouses for life."

At least 38 states and the federal government have approved laws or amendments barring the recognition of gay marriage; last week, the Utah House gave final legislative approval to a measure outlawing same-sex marriages 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.

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.