WASHINGTON -- Igniting a political powder keg, President Bush yesterday strode into the White House Roosevelt Room and insisted that traditional marriage between one man and one woman is in danger and must be defended by changing the U.S. Constitution.

Bush denounced "activist" judges who he said are changing the definition of traditional marriage, which he called "the most enduring human institution.'' He said gay marriage must be banned by amending the Constitution, a laborious and far-from-certain process.

Constitutional amendments require a two-thirds vote in both the House and Senate and then must be ratified by passage in three-quarters of the states, either through their legislatures or by statewide convention, whichever the amendment specifies.

Beyond the 10 amendments that make up the Bill of Rights, which were ratified in 1791, the Constitution has been amended only 17 times in U.S. history. The most recent one, in 1992, specified that a pay hike for members of Congress may not take effect until after an election.

According to the Gallup organization, 51 percent of the public favors an amendment to the Constitution, but about seven in 10 of those surveyed firmly oppose gay marriage. Pollsters say that, in general, Americans fear an epidemic of gay-bashing or an ugly new wave of intolerance but are wary about legalizing gay marriages.

The issue already has become a hot one in the presidential race. The Democratic front-runner, Massachusetts Sen. John Kerry, favors civil unions for gays and lesbians but opposes marriage. But he also opposes Bush's call for a constitutional amendment.

"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,'' Kerry said yesterday.

Bush did not rule out civil unions but also did not answer a question about whether he supports the concept. In fact, he took no questions after his announcement yesterday.

Constitutional scholars say three proposed amendments in Congress would not outlaw civil unions but would leave the matter in the hands of state legislatures. Bush did not endorse any particular language for an amendment.

On Sept. 21, 1996, then-President Bill Clinton signed the Defense of Marriage Act, which passed the House, 342-67, and the Senate, 85-14. It declares that no state must accept another state's definition of marriage.

So far, 38 states have banned gay marriage. But gay marriage opponents are concerned that judges in some states are now approving it.

Bush said in his statement yesterday: "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. Their actions have created confusion on an issue that requires clarity.''

He cited the fact that four Massachusetts Supreme Judicial Court justices in November allowed the commonwealth's attorneys six months to rewrite state law to allow gays and lesbians to marry.

A follow-up opinion from the court earlier this month said that only full marriage rights starting in May, not merely civil unions, would conform to the state Constitution.

In San Francisco, Bush noted, city officials have issued thousands of marriage licenses to same-sex couples despite the California Family Code, which bars them. And he mentioned that Sandoval County in New Mexico last week had also approved some same-sex licenses.

House Speaker Dennis Hastert, R-Ill., said getting a House vote this year on a constitutional amendment banning same-sex marriage was possible. Senate Majority Leader Bill Frist, R-Tenn., said he was dubious that there would be enough time this year to accomplish such a controversial vote, but he said he would try.

Other Republicans had doubts. House Majority Leader Tom DeLay, R-Texas, said changing the Constitution should be approached with caution. And Rep. David Dreier, R-Calif., said:

"I'm not supportive of amending the Constitution on this issue. I believe that this should go through the courts, and I think that we're at a point where [an amendment is] not necessary.''

The conservative Family Research Council praised Bush's announcement, saying, "The president was right to say enough is enough.'' Another group, The Alliance for Marriage, said, "Most Americans believe that gays and lesbians have a right to live as they choose, but they don't have a right to redefine marriage for our entire society.''

But the Log Cabin Republicans, whose members are gay, accused Bush of "pandering to the radical right.''

The Human Rights Campaign, the nation's largest gay and lesbian political organization, said Bush's proposal would be discriminatory as well as "un-American, shameful and divisive."

And Matt Foreman, executive director of the National Gay and Lesbian Task Force, said in a statement, "We consider today's announcement a declaration of war on gay America.''

Massachusetts Sen. Edward Kennedy, a Democrat, said the Constitution traditionally had been amended to expand civil rights, not to discriminate against a segment of society.

Kennedy's comment infuriated Pennsylvania Sen. Rick Santorum, the chamber's No. 3 GOP leader, who has been urging Bush to join the conservatives' bid to change the Constitution. "Marriage is to society as oxygen is to the body. It is the natural law," Santorum said.

"This is not discriminating against anybody. ... Discrimination is so unrelated to this issue [that] it is beyond the pale to discuss it.''

Although there have been rumblings in conservative circles for weeks that Bush was ready to come out in favor of a constitutional change to shore up his conservative base, there had also been some thought that he might decide otherwise because Vice President Dick Cheney has an openly gay daughter.

Cheney has said publicly that he thinks the issue should be decided by the states. But the vice president's supporters yesterday said he had never contemplated that activist judges would approve same-sex marriage licenses on their own and create a chaotic legal situation.

If one state legitimizes same-sex marriage, other states need not recognize the union, but it causes complicated legal ramifications. Santorum said the movement toward allowing same-sex unions had originated in Scandinavia, and that traditional families there are deteriorating as a result.

He said neither gay nor heterosexual Scandinavians are marrying because marriage as an institution has been devalued.

The United States needs a thorough, informed debate on the subject, Santorum said, and if it dominates the presidential race, that's the fault of the judges and "vigilante executives" in local and state government who have been permitting gay marriage recently.

Asked if he were comfortable with constitutional amendment as the best way to resolve the issue, Santorum answered: "The Constitution already has been messed with.

"The right to sodomy is now a constitutional right. I would say that's messing around with Constitution. The only way to stop this is through this messy process I wish I didn't have to go through.''