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. “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. “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 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.