President Bush on Tuesday called for a constitutional amendment to define marriage as a union between a man and a woman, as he expressed alarm at the gay marriages conducted in San Francisco and the potential for same-sex weddings soon in Massachusetts.

In backing an amendment, which his conservative supporters regarded as overdue, Bush thrust the gay marriage issue squarely into the presidential campaign and all but ensured that the controversy would remain alive beyond the November election.

"If we are to prevent the meaning of marriage from being changed forever, our nation must enact a constitutional amendment to protect marriage in America," the president said in a statement that he read at a hastily arranged appearance in the Roosevelt Room of the White House.

He emphasized that he understood the historic consequences of his stance, declaring: "An amendment to the Constitution is never to be undertaken lightly. The amendment process has addressed many serious matters of national concern. And the preservation of marriage rises to this level of national importance."

The president left the room abruptly after his five-minute statement, leaving shouted questions from reporters hanging in the air.

Bush's announcement drew praise from social conservatives, who form the core of the president's political base, and condemnation from advocates for civil rights and gay rights.

The president's call to ban gay marriage came only a day after he abandoned his above-the-fray stance in the presidential campaign and attacked Sen. John F. Kerry of Massachusetts, the front-runner for the Democratic Party's nomination.

Kerry and his leading challenger in the Democratic race, Sen. John Edwards of North Carolina, oppose gay marriages but say the issue should be left to the states. The senators separately accused Bush on Tuesday of playing politics with the controversy.

"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 reelection campaign," Kerry said. He said Bush was "looking for a wedge issue to divide the American people."

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

While the president said the Constitution should restrict marriage to the union of a man and a woman, he also said state legislatures should be free to "make their own choices in defining legal arrangements other than marriage," such as civil unions.

Rep. Marilyn Musgrave (R-Colo.) and 115 co-sponsors already have introduced a proposed amendment in Congress that they say would take that approach.

Civil unions, legal only in Vermont, offer same-sex couples the identical state rights and benefits that married couples enjoy. But because civil unions have no federal standing, same-sex couples cannot take advantage of an array of federal benefits, such as filing joint federal tax returns and receiving survivors' Social Security benefits.

Kerry said Tuesday that while he opposed gay marriage, he saw civil unions as an appropriate way to extend legal protections to gays and lesbians. Bush opposes civil unions, White House Press Secretary Scott McClellan said, although the president believes that states should decide individually whether to allow them.

Historically, proposed constitutional amendments face long odds, even when they have presidential backing. They require the approval of both houses of Congress -- by a two-thirds majority -- and ratification by three-quarters of the state legislatures. Over the years, thousands of amendments have been proposed, but only 17 have overcome those high hurdles since the original 10 were enacted as the Bill of Rights.

On Capitol Hill, Sen. John Cornyn (R-Texas) said his subcommittee of the Senate Judiciary Committee would hold hearings on the issue, starting next week.

"It is very likely that we will act," said Senate Majority Leader Bill Frist (R-Tenn.), although he declined to predict whether the Senate would vote this year on an amendment.

One prominent Republican, Rep. David Dreier of San Dimas, said he would not support an amendment.

"I believe that this should go through the courts," said Dreier, the influential chairman of the House Rules Committee. "We're at a point where it's not necessary, from my perspective."