COLUMBIA, Mo. — First the General Motors assembly plant in Hazelwood, a St. Louis exurb, closed in 2006. The Chrysler plant in south Fenton shuttered two years later, then the plant on the north side of town a year after that.

And then came the Great Recession.

Some 43,000 workers lost their jobs because of the plant closings, according to a study conducted by the St. Louis Economic Development Partnership.

That’s when Missouri conservatives dug in their heels in the fight for so-called “right-to-work” policies, measures that exempt employees from paying mandatory union dues, effectively undercutting a union’s ability to finance itself and hence collectively bargain.

In theory, economists say, such policies should make Missouri a better place for businesses to locate — and bring jobs — because labor would be cheaper and businesses wouldn’t have to deal with the demands of organized labor, an option that looked appealing in the early 2010s when the jobless rate here hovered in the 8-percent range.

So for the last two years, Missouri’s Republican-dominated state legislature has passed right-to-work legislation, only to have it vetoed by Democratic Gov. Jay Nixon.

The back-and-forth made the Show-Me State a national battleground over the future of labor policy and unionism, economists say. Now, Republicans are planning to use the issue to help their party reclaim the governor's mansion; Nixon, a two-term incumbent, is barred by the state's constitution from running for a third term.

Twenty-five states, including six that border Missouri, have right-to-work legislation. Missouri, a blue-collar heartland state with a Democratic governor, could be crucial No. 26, a meaningful bellwether for national conservatives.

“I believe the eyes of the nation are on Missouri next year,” Lt. Gov. Peter Kinder (R), a leading gubernatorial contender, said in an interview. “We need a proven winner who can take this issue and sell it across Missouri.”

Both in labor and politics, Missouri has long been seen as a microcosm of national policy. It’s anchored by two industrial hubs in Kansas City and St. Louis and filled in between with small-town rural communities.

Unions made strong gains throughout the industrial Midwest in the post-World War II era, as returning GIs and their children rushed to fill well-paying jobs, said Nelson Lichtenstein, the director of the Center for the Study of Work, Labor and Democracy at the University of California-Santa Barbara.

In cities like Pittsburgh, those jobs were in the steel industry. In Michigan, it was in automobiles. In Missouri, it was mechanized agriculture, chemical production and eventually auto manufacturing.

In 1950, St. Louis was the eighth most populous city in the country. By 2012, it didn’t crack the top 30.

People left, said Jefferson Cowie, a professor of labor history at Cornell, because the jobs left. Companies found it was easier to send manufacturing jobs overseas where production costs are lower.

Some states, notably in the South, responded by passing right-to-work legislation to hopefully draw manufacturers by making it harder for unions to operate. The entire South, from Texas to Virginia, had passed right-to-work laws by 1976.

The shift had a marked political impact because organized labor tended to cuddle up to Democrats. Republicans came to regard unions as “organizing committees for the Democratic Party,” Lichtenstein said. “It became a bedrock identity issue for Republicans just over decades to be in favor of right to work.”

The jobs that Missouri policymakers are now trying to attract fit outside the traditional union sphere, said Timothy Bartik, a senior economist with the W.E. Upjohn Institute.

The “core economy,” what used to be made up of manufacturing, mining and farming, has been replaced by a new set of industries that are less union-friendly, namely large-scale retail and service sector jobs.

“All the growth is outside the union sectors,” Cowie said. “The old union sectors have fallen apart. Labor has been unable to organize in the new sectors. And if you add that up, it adds up to a dramatic decrease in political power.”

And a boost for Republicans, which now has has deep majorities in both houses of the Missouri state legislature and among voters who consider themselves conservatives.

In 2004, 71 percent of Missourians voted to ban gay marriage by state constitutional amendment. In 2008, Sen. John McCain defeated then-Sen. Barack Obama by a few thousand votes. In 2012, Mitt Romney won every Missouri jurisdiction outside of liberal bastions St. Louis, Kansas City and Boone County, home to the state’s flagship university.

Republican support for right-to-work laws comes as the movement has spread in recent years to the upper Midwest — Wisconsin passed right-to-work legislation this year and Michigan passed it in 2012.

Missouri Republican gubernatorial candidates are using their anti-union stance to set up a fight with likely Democratic nominee Attorney General Chris Koster, who is funded in large part by labor groups and has vowed to block the policy, insisting it hurts wages. An August primary will decide who faces off in the race.

Two days after Republicans failed to override the governor’s most recent veto on right to work, Koster’s campaign received a $20,00 campaign donation from a group representing carpenters. On Sept. 30, he received $300,000 from another labor organization.

Conservatives here trumpet right to work as a way to keep Missouri from falling behind and to attract new jobs. The political clout of unions has waned, they say. The time is ripe for Missouri businesses to shrug them off.

“I think you’re beginning to see a tipping point in the power of unions where they can no longer defend previous gains from the 1930s and 40s,” Cowie said. “Now they’re not even powerful enough to defend themselves against a direct attack."

Correction: A previous version of this story misstated the population ranking for St. Louis in 1950.