The amendment would prohibit states from recognizing same-sex marriages. To become law, the proposal would need two-thirds support in the Senate and House, and then be ratified by at least 38 state legislatures.

It stands little chance of passing the 100-member Senate, where proponents are struggling to get even 50 votes. Several Republicans oppose the measure, and so far only one Democrat - Sen. Ben Nelson of Nebraska - says he will vote for it.

The Senate Judiciary Committee approved the amendment on May 18 along party lines after a shouting match between a Democrat and the chairman, Sen. Arlen Specter, R-Pa. He bid Sen. Russ Feingold, D-Wis., "good riddance" after Feingold declared his opposition to the amendment and his intention to leave the meeting.

Bush aides said he would be making his remarks on the subject Monday.

A slim majority of Americans oppose gay marriage, according to a poll by the Pew Research Center for the People and the Press from March. But the poll also showed attitudes are changing: 63 percent opposed gay marriage in February 2004.

Those poll results don't reflect how people might feel about amending the Constitution to ban gay marriage.

The Massachusetts Supreme Court decided to legalize such marriages in 2003. A year later, San Francisco issued thousands of marriage licenses to gay couples.

This November, initiatives banning same-sex marriages are expected to be on the ballot in Idaho, South Carolina, South Dakota, Tennessee, Virginia and Wisconsin. In 2004, 13 states approved initiatives prohibiting gay marriage or civil unions, with 11 states casting votes on Election Day.

Bush benefited as religious conservatives turned out to vote and helped him defeat Democratic Sen. John Kerry in 2004. In Ohio, an initiative rejecting the legality of civil unions won handily. The same state tipped the election to Bush.

"The president firmly believes that marriage is an enduring and sacred institution between men and women and has supported measures to protect the sanctity of marriage," White House spokesman Ken Lisaius said.

Bush has lost support among conservatives who blame the White House and Congress for runaway government spending, illegal immigration and lack of action on social issues such as the gay marriage amendment.

Opponents of the amendment objected to Bush promoting a measure they said amounts to discrimination.

"This is fundamentally both a civil rights and religious freedom issue and the president's position of supporting amending the constitution is just dead wrong," said Barry W. Lynn, executive director of Americans United for Separation of Church and State. "This is simply to give ammunition to the so-called religious right just to show that the president is still with them."