Each side says the fight was forced upon them by the other, and now the climactic showdown is at hand: Voters in 11 states will decide Tuesday whether to impose constitutional bans on same-sex marriage.

Rarely in American history have so many voters — close to one-fifth of the electorate — had a chance on a single Election Day to express themselves on such a highly contentious social issue. Most, if not all, of the bans are expected to win approval, though national gay-rights groups are spending heavily in Oregon and a few other states in hopes of avoiding a shutout.

"We are huge underdogs in every one of these battles," said Matt Foreman of the National Gay and Lesbian Task Force (search). "Our side simply does not have the time, the resources or the infrastructure to beat back the forces being unleashed against us in this election year."

The 11 ballot items result from a backlash to the court ruling almost a year ago that made Massachusetts the only state with legalized gay marriage. Seeking extra protection against any comparable future rulings, legislators in five states and signature-gathering citizens' groups in six states placed proposed constitutional amendments on Tuesday's ballots that would limit marriage to one man-one woman unions.

"This is an issue that reaches deep and wide across this country," said Tony Perkins, president of the conservative Family Research Council (search). "The people are taking the lead — they're not waiting for the politicians to act."

Conservative leaders say the ballot campaigns were a necessary response to gay-rights lawsuits seeking marriage equality and Congress' failure to pass a federal amendment banning gay marriage — though conservatives plan to raise the amendment issue there again. Many gay-rights activists see the campaigns as a mean-spirited tactic to boost conservative turnout on Election Day at the expense of gay and lesbian couples.

"We were not looking for this fight," said Stacy Fletcher of Arkansans for Human Rights. "There is no gay agenda. All our community was doing was working, paying taxes and trying to live our lives."

The proposed amendments in Mississippi, Montana and Oregon refer only to marriage. Those in Arkansas, Georgia, Kentucky, Michigan, North Dakota, Ohio, Oklahoma and Utah would ban civil unions as well, and those extra provisions have generated extra controversy.

In Utah, for example, all three candidates for attorney general — including incumbent Republican Mark Shurtleff — oppose the amendment because it would forbid granting "the same or substantially equivalent legal effect" as marriage to other relationships. The candidates say this could bar the Legislature from extending even basic partnership rights such as hospital visitation to any unmarried couples.

In Ohio, similar concerns have prompted several top Republicans, including Gov. Bob Taft, to oppose the amendment — even though its presence on the ballot is viewed as a potential benefit to President Bush. In Michigan, another presidential battleground, the state AFL-CIO has condemned the amendment as a threat to domestic partnership benefits offered by public employers.

"The value of this campaign has been the debate itself," said Joan Garry, executive director of the Gay and Lesbian Alliance Against Defamation. "You don't ever change people's attitudes about something they feel really strongly about without arguing like hell."

Already this year, voters in Missouri and Louisiana have weighed in on the issue, with marriage amendments winning more than 70 percent of the vote in both states. Louisiana's amendment was later struck down in state court.

Recent polls showed support for the amendments at 76 percent in Oklahoma and Kentucky, 65 percent in Arkansas, 60 percent or more in Michigan, 59 percent in Montana, 57 percent in Ohio.

In Oregon, with perhaps the tightest campaign, some amendment supporters assert that public schools would be forced to provide positive instruction about same-sex marriage unless the ban is imposed. The claims angered the state school superintendent.

"They have no business using our public schools as part of this campaign," said Susan Castillo. "They're trying to create some sort of fear."

The Rev. B.G. Stumberg, pastor of a Baptist church near Helena, Mont., contends the bans are essential to America's future. "If you destroy the family, you're going to rock the basis of what society is all about," he said.

The National Gay and Lesbian Task Force and the Washington-based Human Rights Campaign have each donated more than $1 million to anti-amendment campaigns, mostly in Oregon, Ohio and Michigan. Much of the money supporting the amendments has come from churches; in Michigan, every Roman Catholic diocese has contributed.

Georgia's Catholic bishops have directed parishioners to vote for the amendment. In Utah, the powerful Mormon church extolled heterosexual marriage and declared that "any other sexual relations, including between persons of the same gender, undermine the divinely created institution of family."