WASHINGTON (AP) – President Bush rallied support Monday for a ban on gay marriage as the Senate opened a volatile, election-year debate on a constitutional amendment to prohibit same-sex weddings.

“Changing the definition of marriage would undermine the structure of the family,” said Bush, who raised the issue’s profile with an event at the Eisenhower Executive Office Building.

Bush criticized judges who have overturned state laws similar in intent to the proposed legislation. “Marriage is the most fundamental institution of civilization, and it should not be redefined by activist judges,” he said.

Traditional marriage, Bush said, is the cornerstone of a healthy society and the issue should be put “back where it belongs: in the hands of the American people.”

There was little chance of that in the near future. Neither chamber is likely to pass the amendment by the two-thirds majority required to send it to the states _ three quarters of which would then have to approve it.

Many Republicans support the measure because they say traditional marriage strengthens society; others don’t but concede the reality of election-year politics.

“Marriage between one man and one woman does a better job protecting children better than any other institution humankind has devised,” said Senate Majority Leader Bill Frist, R-Tenn. “As such, marriage as an institution should be protected, not redefined.”

Judiciary Committee Chairman Arlen Specter, R-Pa., said he will vote against the measure on the floor but allowed it to get there in part to give the GOP the debate party leaders believe will pay off on Election Day. Specter has chosen a different battle with the Bush administration this week _ a hearing Tuesday on the ways the FBI spies on journalists who publish classified information.

As that hearing gets under way, debate on the marriage amendment will enter its second day on the Senate floor. All but one of the Senate Democrats _ the exception is Ben Nelson of Nebraska _ oppose the measure and, with moderate Republicans, are expected to block an up-or-down vote, killing the measure for the year.

Democrats say the amendment is a divisive bow to religious conservatives, and point out that it conflicts with the GOP’s opposition to big government interference.

“A vote for this amendment is a vote for bigotry pure and simple,” said Democratic Sen. Edward M. Kennedy of Massachusetts, where the state Supreme Court legalized gay marriages in 2003.

Senate Democratic Leader Harry Reid of Nevada, who says he believes marriage is the union of a man and a woman, said he nonetheless will vote against the amendment on a test vote Wednesday.

“The reason for this debate is to divide our society, to pit one against another,” Reid said in remarks prepared for delivery on the Senate floor. “This is another one of the presidents efforts to frighten, to distort, to distract, and to confuse America. It is this administration’s way of avoiding the tough, real problems that American citizens are confronted with each and every day.”

Mayor Gavin Newsom of San Francisco, which in 2004 began issuing marriage licenses to gay couples, on Monday denounced Bush’s move as predictable and “stale rhetoric” aimed at rallying conservatives for this year’s midterm elections.

“It’s politics. It’s pandering and it’s placating a core constituency, the evangelicals,” Newsom said on ABC’s “Good Morning America.”

White House press secretary Tony Snow acknowledged said there was a political dimension to Bush’s remarks Monday, but said the president was not calling senators to persuade them to pass the amendment.

“I’m not sure this is a big driver among voters,” Snow said.

Bush’s views on the federal marriage amendment differ from those held by Vice President Dick Cheney, whose daughter, Mary, is a lesbian.

Cheney said he thinks Americans should do everything they can to tolerate and accommodate whatever kind of relationships people want to enter in to. He said he does not think there should necessarily be a federal policy in this area.