President Bush called Tuesday for a constitutional amendment to bar gay marriage (search), saying the nation must defend "the most fundamental institution of civilization."

"If we are to prevent the meaning of marriage from changing forever we must" pass a constitutional amendment, Bush said from the Roosevelt Room at the White House.

Bush said he was troubled by the decisions "a few judges and local officials" were making in favor of gay marriage in San Francisco, Massachusetts and elsewhere. San Francisco has permitted more than 3,000 same-sex unions in a display of wide-scale civil disobedience.

"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. "Their action has created confusion on an issue that requires clarity."

Although Bush said he wanted the Constitution amended to preserve the definition of marriage as being between one man and one woman, he said that states should be allowed to craft laws that recognize other types of relationships.

Up until this point, Bush was a staunch opponent to the idea of same-sex marriage, but he had reserved judgment on the need for an amendment. He changed his position because of the recent legal moves.

"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.

To change the Constitution, a two-thirds majority of both the House and Senate needs to pass the proposed amendment. From there, two-thirds of all state legislatures needs to ratify the amendment.

It won't be an easy task. Two senior House Republicans — California Reps. David Dreier (search) and Jerry Lewis (search) — 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."

Just Right or Too Far?

Bush's announcement was hailed by many Republicans and conservative organizations, but Democrats — even those who oppose gay marriage — said the call to amend the Constitution went too far.

"I agree with the president in terms of being opposed to same-sex marriage, but to tamper with the Constitution on an issue like this that should be left up the states" is a mistake, New Mexico Gov. Bill Richardson (search) told Fox News.

The marriage issue is likely to play a prominent role during this year's presidential election. Bush's main Democratic rivals do not support marriages between two men or two women, but they have opposed taking the constitutional route.

Sen. John Kerry (search), who hopes to run against Bush in this year's presidential election, said: "I believe President Bush is wrong."

The Massachusetts Democrat said he prefers civil unions and rejects any federal or state legislation that could be used to eliminate equal protections for homosexuals or other forms of recognition like civil unions.

"I oppose gay marriage. I also oppose President Bush's attempt to amend the Constitution to ban gay marriage," said Sen. John Edwards (search), D-N.C., Kerry's chief rival in the race for the Democratic presidential nomination.

The Democratic National Committee called the decision purely political. "It is shameful to use attacks against gay and lesbian families as an election strategy," DNC Chairman Terence McAuliffe said.

In a new poll, just 41 percent of respondents said they would favor an amendment to the Constitution barring any state from legalizing same-sex marriages. Forty-eight percent would oppose such an amendment, according to a poll of 1,943 people between Feb. 14 and Feb. 23 by the University of Pennsylvania's National Annenberg Election Survey (search).

However, when it comes to state laws, just 30 percent would favor a law allowing gays and lesbians to marry a partner of the same sex, while 64 percent would oppose it.

Crafting an Amendment

Just how a constitutional amendment would look is unclear as several ideas have been floated, including one that would prevent any type of recognition of same-sex relationships as well as one that just defines who can marry.

One option, submitted by Rep. Marilyn Musgrave (search), R-Colo., appears to be the chief option.

"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," the Musgrave amendment states.

White House Press Secretary Scott McClellan (search) said the Musgrave approach meets Bush's "principles" in protecting the "sanctity of marriage" between men and women, but McClellan said Bush was not endorsing a specific approach.

Rep. Charlie Norwood, R-Ga., a cosponsor to Musgrave's legislation, praised Bush's support for a constitutional amendment.

"President Bush is absolutely right to stand up, show his support for a constitutional amendment to defend marriage, and send a strong message to that crowd," Norwood commented in a statement on Tuesday.

Bush decided to take action partly because the Massachusetts Supreme Judicial Court recently ruled that it is unconstitutional to bar gay couples from marriage. That decision could result in gay weddings there as early as May, McClellan said. "We're two months away," he said.

McClellan said 38 states have passed laws protecting the "sanctity of marriage," and the president will call on Congress to move quickly to pass legislation that can then be sent to the states for ratification. 

"We need to act now," he said. "The constitutional process will take time." Passing a constitutional amendment is a difficult and lengthy process. Winning the support of two-thirds of the House of Representatives, two-thirds of the Senate and ratification by three-quarters of the states can take years.

Interest Groups React to Bush's Decision

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

A major gay Republican group, the Log Cabin Republicans (search), 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 American Center for Law and Justice (search), 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 one man and one woman."

Fox News' Peter Brownfeld and The Associated Press contributed to this report.