Updated

The presidential race is not the only high-stakes political battle being waged in Missouri this year.

The state votes Tuesday on a constitutional amendment to ban gay marriage (search), marking the first such vote in the nation since Massachusetts legalized same-sex marriage last year. National groups on both sides of the debate expect the vote to be a litmus test for which campaign strategies work — and which don't — as the battle spreads to ballot boxes around the United States.

At least nine other states, and perhaps as many as 12, will vote on similar amendments this year. Four states already have similar amendments.

"What happens here in Missouri could have a tremendous impact on the rest of the nation's other elections," said Vicky Hartzler, spokeswoman for the group backing the amendment, the Coalition to Protect Marriage in Missouri (search). "I'm hopeful we will be able to send a very strong, clear message from Missouri that here in the heartland, we value traditional marriage."

The proposed amendment has prompted national gay-rights groups to send more than $100,000 to the Missouri organization fighting the ban, and they expect to spend millions of dollars around the country before the general election.

A recent poll showed that about 60 percent of Missouri voters support the amendment. Gay rights supporters acknowledge that it is an uphill fight, but they believe the state at least will help the movement figure out what strategies work as it gears up for elections elsewhere.

"Missouri is the first one. It's really important that the gay and lesbian community and our allies wage as strong a campaign as we can to send a strong message to the other campaigns that are going on out there," said Seth Kilbourn, national field director for the Human Rights Campaign (search), a gay rights organization.

The debate intensified last year after a landmark court decision in Massachusetts cleared the way for same-sex weddings there, prompting several states to push for constitutional amendments that ban gay marriage.

Missouri and 37 other states already have laws that define marriage as solely between a man and a woman, but supporters of the amendments fear the laws are susceptible to being tossed by a court.

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. There also are initiatives pending in Michigan, North Dakota and Ohio.