Missouri voters solidly endorsed a state constitutional amendment banning gay marriage, a decision that was closely watched by national groups on both sides of the battle.

With nearly all precincts reporting, the amendment had garnered 71 percent of the vote, according to unofficial results for Tuesday's vote. It was the first such vote since the historic ruling in Massachusetts last year that legalized same-sex weddings (search) there.

Although the ban was widely expected to pass in conservative Missouri, experts said the campaign served as a key barometer for which strategies work as at least nine other states, and perhaps as many as 12, vote on similar amendments this year.

Missouri and 37 other states already have laws defining marriage as only between a man and a woman. But amendment supporters fear a court could toss aside the state law, and they believe the state would be on firmer legal ground if an outright ban is part of the Constitution.

"I'm very gratified and encouraged and thankful that the people of this state understand our current policy's a wise public policy and they want to see it protected from a legal challenge," said Vicky Hartzler, a spokeswoman for the Coalition to Protect Marriage in Missouri (search).

Opponents said the amendment was unnecessary and discriminatory, but knew they faced an uphill battle in Missouri.

"We're already reaching out to these other states, sharing with them what we learned, what worked, what didn't work, and we'll move on," said Doug Gray, campaign manager for the Constitution Defense League (search). "Ultimately we're right and they're simply wrong."

Supporters and opponents of the amendment have used grassroots campaigns, knocking on doors and making phone calls to tell people about the issue. The group fighting the amendment, the Constitution Defense League, raised more than $360,000, largely from national gay-rights groups, and ran a television ad in the final days before the vote.

The group favoring the amendment, the Coalition to Protect Marriage in Missouri, spread the word through churches and community events, raising just a few thousand dollars but saying public sentiment in Missouri was on their side.

Louisiana residents are to vote on a marriage amendment Sept. 18. Then Arkansas, Georgia, Kentucky, Mississippi, Montana, Oklahoma, Oregon and Utah are to vote on the issue Nov. 2. Initiatives are pending in Michigan, North Dakota and Ohio.

Four states — Alaska, Hawaii, Nebraska and Nevada — already have similar amendments in their constitutions.