Washington ? President Bush urged approval of a constitutional amendment banning gay marriages on Tuesday, pushing a divisive social issue to the center of the election campaign and setting a clear policy contrast with Democratic challengers John Kerry and John Edwards.

Bush said “activist judges and local officials” from Massachusetts to San Francisco to New Mexico were attempting to redefine marriage and “change the most fundamental institution of civilization” by allowing same-sex weddings. “On a matter of such importance, the voice of the people must be heard,” he said.

Democrats accused Bush of pandering to right-wing supporters and tinkering with the Constitution to divert attention from his record on jobs, health care and foreign policy. “He is looking for a wedge issue to divide the American people,” Kerry said.

Both Kerry and Edwards said they oppose gay marriages but would not support a constitutional amendment.

Banning gay marriage is a top priority for Bush’s conservative supporters, particularly those among religious and family-oriented groups. But while a majority of Americans — sometimes by as much as a 2-1 margin — oppose legalizing gay marriages, Bush’s move could hold political risks, particularly if voters see him as intolerant or question his self-description as a “compassionate conservative.”

“The president needs to worry about fair-minded swing voters in America, not a Republican base that he has locked up,” said Patrick Guerriero, executive director of Log Cabin Republicans, a gay GOP group.

Mindful of the high emotions and clear differences on the issue, Bush said, “We should also conduct this difficult debate in a manner worthy of our country, without bitterness or anger.”

Conservatives were delighted Bush had plunged in. “There is no more important issue for our nation than the preservation of the institution of marriage,” said Kelly Shackelford, president of the Texas-based Free Market Foundation, a family advocacy group.

Momentum for a constitutional amendment has grown as San Francisco officials have performed thousands of same-sex marriages and have challenged their state law barring such unions. In Massachusetts, the state’s highest court has ruled that a state law falling short of allowing full-fledged marriage for gays would be unconstitutional.

Bush softened his announcement by leaving the door open for states to legalize civil unions, which gay rights groups say is an insufficient alternative to marriage. “The amendment should fully protect marriage while leaving the state legislatures free to make their own choices in defining legal arrangements other than marriage,” said Bush, who had opposed legalizing civil unions when he was governor of Texas.

No GOP consensus

Republican officials said there was no rush to bring an amendment to the floor in the House. Some conservatives want a broader approach than Bush supports, and others oppose federalizing the issue, at least for now.

“The groups that are for a constitutional amendment are split over what it should be,” said House Majority Leader Tom DeLay, R-Texas. “We are trying to bring them all together and unify them.”

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 changing the Constitution should be a last resort on almost any issue.

Democrats weigh in

House Minority Leader Nancy Pelosi, a Democrat from San Francisco, said she would fight any amendment. “Never before has a constitutional amendment been used to discriminate against a group of people, and we must not start now,” she said.

Bush’s call for a gay-marriage amendment came as the president sought to regain his footing after he was thrown on the defensive about issues ranging from his Vietnam-era military record to missing weapons of mass destruction in Iraq.

“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.

Answering Bush, Kerry said, “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.”

“I believe the best way to protect gays and lesbians is through civil unions,” Kerry said. “I believe the issue of marriage should be left to the states”

Edwards, campaigning in Georgia, where the state legislature is debating its own ban on gay marriage, said, “I don’t personally support gay marriage myself. My position has always been that it’s for the states to decide.”