A national debate over whether homosexuals should be allowed to marry one another intensified Tuesday when President Bush announced that he favors a constitutional amendment that would prohibit same-sex marriages. The question of gay marriages could become a central issue in the U.S. presidential election this year.

The president inserted himself into the middle of a volatile and divisive debate over gay marriage with his announcement at the White House Tuesday that he has decided to back a constitutional amendment that would ban same-sex marriages.

"The voice of the people must be heard," said Mr. Bush. "Activist courts have left the people with one recourse. 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 the amendment is needed to stop judges from changing the definition of what he called the most enduring human institution. He also said the marriage of a man and a woman cannot be severed from what he said were its cultural, religious and natural roots.

The president's announcement brought a swift and negative reaction from homosexual rights activists around the country.

"For the families that we serve, this is a nuclear bomb," said David Buckel, an attorney with the Lambda Legal Defense Fund, a homosexual rights group. "This can potentially wipe off the map their efforts to try to get equality for their families."

The president's decision came in the wake of a lot of recent activity on the gay marriage issue. Massachusetts officials are expected to begin issuing marriage licenses to gays and lesbians in May at the request of the state supreme court.

And in San Francisco, more than 3,200 same-sex couples have been married since the city began issuing marriage licenses earlier this month, including this woman who says society has little to fear from homosexuals marrying one another.

"By the time a constitutional amendment comes before the people," she said, "all of our marriages will have been legal for several years and people are going to see that there was no reason to fear our marriages."

The issue is sure to become a major subject of discussion in this year's presidential campaign. Massachusetts Senator John Kerry, the front-runner for the Democratic Party's presidential nomination, says the president is trying to tamper with the Constitution because he is in political trouble.

Senator Edward Kennedy, also from Massachusetts, indicated that Democrats in Congress are not anxious to begin work on the lengthy process of passing a constitutional amendment. "I hope we can all agree that Congress has more pressing challenges to consider than a divisive, discriminatory constitutional amendment that responds to a non-existent problem," he said.

But the president's decision to support an amendment opposing gay marriage was hailed by conservative groups, which have been pressing for his backing for months. "Homosexuals are free to do whatever they want," said the Reverend Louis Sheldon, founder and president of the Traditional Values Coalition. "But they are not free to steal an institution [that has been around] since the dawn of history and call it marriage."

Public opinion polls indicate most Americans oppose gay marriage, often by a margin of two to one. Those same polls suggest the public is more divided over whether to recognize so called civil unions in which homosexual couples are granted most of the legal rights and privileges given to married heterosexual couples.

At least 38 states and the federal government have approved laws or amendments barring the recognition of gay marriages. Vermont recognizes civil unions among gays and lesbians, and a few other states have passed laws that extend some rights to spouses of same-sex marriages.

Conservative leaders say they believe the gay marriage issue could benefit President Bush and other Republican candidates in the November election. Brian Brown is with a group called the Family Institute of Connecticut.

"The people of this country oppose this, even in Massachusetts," said Mr. Brown. "And so this sort of leadership is very needed and I think President Bush is just doing what the people of this country want him to do, which is to stand up in defense of marriage."

But even supporters of the idea acknowledge that passing a constitutional amendment is a daunting task. Two-thirds of both the House of Representatives and the Senate would have to approve the amendment as well as three-quarters of the 50 states, a process that would likely take years.