After proceeding cautiously for weeks, the Republican-led Congress is moving full-speed on proposals to bar same-sex marriage even though leaders in both houses acknowledge they lack the votes to pass them.

The quickened pace on a doomed but emotionally divisive legislative issue comes as Democrats prepare for their national convention this month and enjoy a bounce from this week's vice presidential pick. Some Democrats say the Republicans' insistence on holding a vote they know they will lose is a political ploy to energize conservatives and possibly appeal to swing voters.

Polls show that most Americans oppose gay marriage but are deeply divided on whether a constitutional ban is needed. Some congressional Republicans have urged their leaders to move cautiously lest the party be accused of gay-bashing or opportunism.

House Minority Leader Nancy Pelosi (D-Calif.) made just such an accusation yesterday. "That they would move forward with an amendment to the Constitution that they knew was doomed to fail," she told reporters, is "red meat for their constituents. They find it necessary because even their own constituents are catching on" to problems in the economy and elsewhere.

The Senate plans to start debate today on a constitutional amendment to bar same-sex marriages. A Republican leadership aide said the proposal's backers have "in the neighborhood" of 50 votes, while a Democratic aide said the number is closer to 40 or 45.

Both estimates fall well short of the 67 votes that would be required if all senators are present. For a proposed constitutional amendment to be sent to the states for ratification, the House and Senate each must give it a two-thirds majority vote.

The issue's accelerated pace is especially dramatic in the House. Two weeks ago Majority Leader Tom DeLay (R-Tex.) said he did not want the House or Senate to vote on a constitutional amendment until they had enough votes to pass it.

But he told reporters Wednesday that the traditional notion of marriage "is under attack" and "we intend to fight it on all fronts" this summer. Before Congress adjourns July 23 for a five-week recess, DeLay said, the House will vote on a bill that would bar federal courts from ruling on state laws defining marriage.

"At the same time, we expect to go forward sometime in September" with a vote on a constitutional amendment similar or identical to the Senate version, DeLay said. He said he had "no idea" whether the 435-member House could muster the necessary two-thirds majority. Some senior Republicans say the goal is unattainable.

"I'm not even sure they can get a simple majority," Rep. Thomas M. Davis III (R-Va.) said yesterday. Davis said he and numerous GOP colleagues will vote against the proposed constitutional amendment, as will most House Democrats.

President Bush supports a constitutional ban on same-sex marriages. Democratic presidential candidate John F. Kerry supports civil unions, but not marriage, for gay couples, and opposes a constitutional amendment. The issue is prominent in some congressional races, such as the Senate race in South Dakota, where Republican John Thune is pressing Senate Minority Leader Thomas A. Daschle to support a constitutional amendment. Daschle, like Kerry, has said states should deal with the issue.

DeLay spokesman Stuart Roy said the House will act on the proposed constitutional change even if it lacks a two-thirds majority because "voters want people to be on the record" on the gay marriage issue. Asked why the vote would be held in September -- when the presidential and congressional election campaigns will hit full stride -- Roy said, "you want to have a vote on it while there's a window of opportunity" in which Americans are paying attention to politics and public issues.

Roy said DeLay in general "does not like amending the Constitution," but feels it is necessary to ensure that marriage involves only "one man and one woman." Massachusetts's highest court last year struck down the state's opposite-sex-only marriage laws, and same-sex marriages have been conducted there and in San Francisco and a few other cities.

A House subcommittee soon will take up a bill, sponsored by Rep. John N. Hostettler (R-Ind.), that would bar federal courts from overturning state laws that ban same-sex marriages. Whether it passes or fails, Roy said, the bill will serve "to build momentum" for the September debate and vote on the proposed constitutional amendment.

In a recent Washington Post-ABC News poll, 59 percent of Americans said it should be illegal for gay couples to marry, with nearly half of all respondents saying they felt "strongly" about the issue. But 53 percent said state laws should govern such questions, while 44 percent said a constitutional amendment is needed.

Republican leaders deny political motives in pushing for a vote they expect to lose, saying they believe it is important for elected legislators, not judges, to set policy on same-sex marriages.

"Activist judges and lawyers have been working to redefine traditional marriage in literally dozens of states," said Sen. Wayne Allard (R-Colo.), chief sponsor of the marriage amendment in the Senate. Asked why Republicans were pushing for a vote before lining up the needed support, Allard said, "You've got to get started sometime. . . . I think we need to have a vote this year because of the urgency of the issue and the need for people to understand it."