Legislation limiting the power of unions is headed to Michigan Gov. Rick Snyder's desk, where the Republican will almost certainly sign "right to work" into law.

But union organizers say they can still undo the contentious legislation, which bars the mandatory collection of labor dues. 

Michigan Republicans attached a spending measure to the bill, which means it cannot be undone by referendum. However, unions say that they can still undo the new law by statutory initiative.

Under this approach, according to the Citizens Research Council of Michigan, opponents of the law would have to file petitions with signatures of registered voters equal to eight percent of the total votes cast in the last gubernatorial election. The legislature would then either enact or reject the petition -- presumably the latter. After that, it would go on the ballot for the next general election, in 2014. 

Unions are already gearing up for a major fight against Snyder in 2014, when his first term ends. Under this plan, they could push for his defeat and the defeat of "right to work" at the same time. 

“If this bill is signed today, it will be Thunderdome for Governor Snyder and Michigan for the next two years," said one high-ranking labor official who requested anonymity to discuss strategy. "There are multiple options for a referendum. The voters will have their say on this issue, and all options are on the table. This fight is far from over.”

Unions have also suggestedthat they might target Republican legislators in recalls next year. The Republican majority in the House is shrinking, but the GOP will still have a 59 to 51 seat edge. In the Senate, it's a 26 to 12 seat GOP majority. 

"We are considering all options that are on the table but whether it is the available ballot initiative option, or Snyder's re-election itself, he will strongly hear the voices of Michiganders in 2014," said Eddie Vale, spokesman for the labor super PAC Worker's Voice.

In the meantime, the law would take effect. But union officials say that the damage might not be so great, because existing contracts and the dues arrangements in them are exempted from the new law. The United Auto Workers contract, for example, is not up again until 2015.