Washington ? Gay marriage has been the sleeper issue of the election year thus far, but a surprising surge of gay-marriage opponents to the polls in Missouri on Tuesday suggests that an issue that went nowhere in Congress is resonating in the country. It could spread to other states planning referenda and ultimately could help President Bush’s re-election prospects.

Missourians voted by a lopsided 71 percent to 29 percent vote to amend their state constitution to define marriage as between a man and woman only.

Voter turnout for the Missouri election exceeded forecasts by as many as 400,000. The big increase, in a primary dominated by Democrats, helped advocates of the amendment leap past a better-financed opposition and win.

Though it was the fifth state to add a ban against same-sex marriage to its constitution, Missouri was the first to do so since a Massachusetts court ruled last year that its state-mandated ban was unconstitutional. Missouri also offered the first popular test of the issue in a large state balanced almost evenly between Republicans and Democrats. And it was the first time voters got to weigh in since the U.S. Senate rejected a proposed marriage amendment to the U.S. Constitution on July 14.

“This vote reveals that support for traditional marriage is strong across party lines,” said Robert Knight, director of the Culture and Family Institute at Concerned Women for America, a conservative Washington-based group. “We expect similar victories in other states beginning with Louisiana in September, and then following through to 10 other states or more.”

In addition to Louisiana, the nine other states that will vote on marriage amendments this fall are Arkansas, Georgia, Kentucky, Michigan, Mississippi, Montana, Oklahoma, Oregon and Utah. Two more, North Dakota and Ohio, could vote on amendments if petitions to get the measure on the ballot are certified.

Fifteen state legislatures, including Kansas, have rejected proposed marriage amendments to their constitutions.

The lineup of states voting this fall includes several closely divided political battlegrounds such as Arkansas, Michigan and Ohio that could decide the presidential election.

As Missouri demonstrated, the question of marriage on the ballot could draw more voters to the polls. Given strong majority sentiment nationwide against gay marriage, those additional voters are likely to be opponents of gay marriage and supporters of President Bush. He has endorsed amending the U.S. Constitution to ban gay marriage.

At the same time, a heated debate over marriage could strain the Democratic base.

“The marriage issue cuts across the Democratic coalition,” said John Green, a political scientist at the University of Akron in Ohio. “Working-class Democrats, rural Democrats, African-Americans and Hispanics tend to have conservative views on marriage. To the extent this issue becomes front and center, it could pull the coalition in different directions.”

That helps explain why Democrats don’t like to talk about the issue. They all but ignored it at their national convention last week in Boston.

Democratic presidential candidate John Kerry opposes gay marriage. He has said he would support a Massachusetts state constitutional amendment to ban gay marriage if it permitted civil unions by same-sex couples. He opposes a proposed same-sex marriage ban amendment to the U.S. Constitution. The Democratic Party platform concludes that “marriage has been defined at the state level for 200 years and we believe it should continue to be defined there.”

The Missouri vote was not surprising. Pre-election polls showed about 60 percent of the voters approved of the amendment. National polls show that about 60 percent of Americans oppose gay marriage.

But the big turnout did surprise analysts. One reason was that advocates of the amendment ran a largely under-the-radar campaign that spent less than $10,000, using word of mouth and help from church-based networks rather than expensive advertising. Opponents of the amendment spent more than $360,000, including more than $100,000 from national gay-rights groups.