"Fundamentally, in the public sector, the resentment to unions is about taxes," said Henry Farber, an economics professor at Princeton University. "In the private sector, it's about international trade and about companies wanting higher profits."
The efforts to weaken both public- and private-sector unions are expected to continue in coming months, as experts predict that those who oppose unions work to curb their power in other traditionally labor-friendly states.
About 14.8 million people, or 11.8 percent of U.S. workers, belonged to a union in 2011, according to the Bureau of Labor Statistics. Of that group, about 7.6 million were public-sector workers, while about 7.2 million worked for the private sector.
The ranks of private-sector unions have been decimated over the past few decades by losses in traditionally union-heavy industries like manufacturing. But union membership among government workers has held relatively steady.
About 37 percent of public-sector workers belonged to a union in 2011, compared with just 6.9 percent of private-sector workers.
Politicians who have taken on public-sector unions say the cost of providing government workers with the salaries, health benefits and pensions they were promised has simply become too high.
Wisconsin Gov. Scott Walker, whose successful effort to curtail collective bargaining rights was met with fierce protests and even a recall effort, said he was motivated by state budget woes.
"I'm just trying to balance my budget," Gov. Walker, a Republican, told The New York Times in 2011.
But others argue that these battles are really about curbing union clout and resources, and note that unions have traditionally been powerful Democratic allies at election time.
"The fight over right-to-work and collective bargaining is really, I think, designed to take away the power that public employees' unions have over the ballot box," said Michael Hicks, director of the Center for Business and Economic Research at Ball State University.
Experts say there are legitimate concerns about how state governments will be able to pay for the benefits they have promised workers, especially as the population ages and retirees live longer.
Matthew Finkin, a law professor at the University of Illinois and an expert in labor issues, said that over the years a common tactic for keeping government workers happy while also balancing state budgets has been to offer longer-term benefits, such as health care for retirees, in lieu of perks that would be costly in the short-term, such as big salary bumps.
That also was an attractive option to politicians, he said, because chances were low that they would still be in office when the bill actually came due.
"It's cheap for this legislature to let some other legislature 10 years from now worry about (it)," Finkin said "They can just push these costs — kick the can, as they say, down the road. Well, you can't do that forever."
The recession and weak recovery have been rough on many state and local budgets, thanks to years of high unemployment and the housing bust. Meanwhile, some believe the gap between what governments have promised workers and what they actually have set aside has topped $1 trillion.
For the 2010 fiscal year, the Pew Research Center estimates that states have set aside about $757 billion less than they need for pension obligations, and about $627 billion less than they need for promised health benefits.
Joshua Rauh, a finance professor at Stanford University, argues that the gap could actually be around four times higher than what Pew estimates. He said that's because many states use very optimistic models for forecasting their expected returns on investments.
Union advocates like Van Roekel, the president of the NEA, argue that the state governments got themselves in trouble by not setting aside enough money to pay for the things they promised. He argues workers have paid the share they agreed to and should not be asked to do more.
"I think they have some legitimate problems that were created by not putting in what they should every year," Van Roekel said. "But that doesn't mean (the problems) can't be solved."
But not everyone thinks the financial challenges facing state and local governments can be fixed without workers making a sacrifice. The most high-profile fights have come in states like Wisconsin, which passed legislation severely limiting union workers' bargaining rights, and Ohio, where similar legislation ended up being repealed by a public vote.
Those challenges, along with the anti-union legislation passed in Michigan last week, were backed by Republicans. But in Rhode Island, a Democratic general treasurer, Gina Raimondo, has led an effort to overhaul the state's pension system. Several unions have challenged the changes in court.