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 the gay marriage battle spreads to ballot boxes around the nation. At least nine other states, and perhaps as many as 12, will vote on similar amendments this year.
The amendment had garnered 71 percent of the vote with 97 percent of precincts reporting.
Show-me state voters were making news on another front as well. Democrats sent Gov. Bob Holden packing. He lost to Democratic Auditor Claire McCaskill. Come November, McCaskill will face Republican Secretary of State Matt Blunt, who handily turned back five lesser-known opponents to win the Republican primary.
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 state 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.
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. "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 already have similar amendments.
In the Democratic primary, Holden's stunning loss capped a term mired in turmoil since his first day in office. He was derisively dubbed "One Term Bob" by opponents.
"Tonight is the beginning of the Missouri comeback," McCaskill declared during a victory party in Kansas City. "Tonight is the night that, as Democrats, we can focus on hope and have confidence that we can win in November."