When Michigan became the 24th right-to-work state in December 2012, conservatives hailed it as a sign the Rust Belt state was "open for business," union workers decried the law as "union busting" and labor leaders vowed to use it to quash Republican Governor Rick Snyder's reelection bid in 2014.

A year later, the impact of the law remains up for debate.

Conservatives say it has already made Michigan more attractive to businesses, but it could take years before the economic benefits can be measured. Unions and Democratic opponents are pronouncing it a failure because unemployment has gone up. And in the gubernatorial race, it does not seem to be a central issue - at least not yet.

Michigan's right-to-work law, which bans making union membership a condition of employment, is similar to those already in effect in 23 other states. Except for Indiana, which also passed a right-to-work law last year, laws in the other states have been in effect for years. Michigan stands apart because it is home to the United Auto Workers, the center of the U.S. auto industry, and a swing state in national elections, where worker rights can become polarizing issues.

"Right-to-work has done none of the things its backers promised," said Bill Black, Michigan state legal and political director for the Teamsters union. "It has not brought better jobs and it has not helped the people of Michigan."

But people outside the state are taking a different view. Tracy Bosman, a Chicago-based site selection consultant with Biggins Lacy Shapiro & Co, says Michigan's law has generated interest in the state.

Up to 50 percent of manufacturers automatically screen out any non-right-to-work state, Bosman said, so Michigan was out of the picture for many companies looking to add production capacity.

"While it does not guarantee success for Michigan, it does at least mean the state will get a second look from firms that automatically excluded it in the past," she said.

Union leaders say the Michigan law has not hit membership numbers since it took effect in March. Unions have sought to persuade the rank-and-file that membership has value, with targeted marketing that included meeting with members statewide.