On Tuesday, Michigan became the 24th state in the nation to enact a "right-to-work" law designed to weaken labor unions. Under the new rules, employees in unionized workplaces will no longer be required to pay unions for the cost of being represented. (Workers could already choose not to join the union.) Protests flared throughout the day, but in the end Michigan Gov. Rick Snyder (R) signed the bill.

Why does this matter? To get a better sense for the history of these right-to-work laws — and what they portend for the future of organized labor — I called Nelson Lichtenstein, a labor historian at University of California, Santa Barbara. His basic take: These laws drive a stake through the concept of solidarity, which is essential for unions to thrive.

Brad Plumer: Where did these "right-to-work" laws come from? And when did conservatives start using them in order to weaken labor unions? Walk me through the history here.

Nelson Lichtenstein: Go back to the 1930s, when the Wagner Act was created to allow the government to certify voluntary unions as the exclusive bargaining agent for a workplace. There was one thing the Wagner Act didn't say anything about. If you're the exclusive representation in a workplace, what about workers who don't want to join? And employers could be quite clever in winning workers away from the union.

So John L. Lewis and other unionists said we need to have a union shop. The employer hires the worker, and after they get the job, they have to join the union. (This was different from the closed shop, where if you want a job, you go to the union first.) Companies resisted this idea bitterly. But then World War II comes along. Unions are going to be patriotic, accept a no-strike pledge, let the government set wage levels. And in return, the government said, okay, we'll give you the union shop. And union membership went up by millions of workers.

But at this very moment, conservatives were already mobilizing against the New Deal and unions. They were saying, we want to pass laws that will make a collective bargaining contract illegal if joining a union becomes a condition of employment.

BP: So already in the 1940s you're seeing a right-to-work movement form—conservatives want rules that forbid employers and unions from agreeing to a union shop arrangement. 

NL: Right, the language on this goes all the way back to the 19th century. The guy who invented the term "right-to-work," I think that was invented in the Progressive Era. What's remarkable is that anti-union discourse hasn't changed over the years. It doesn't matter whether unions are weak or strong. The talk is always about union bosses, about coercion, about how there should be freedom in the workplace.

BP: But when did right-to-work laws start emerging in the states?

NL: In 1947, the Taft-Hartley Act is passed by a conservative Congress, and one of the things it does is to authorize states to pass right-to-work laws. The Southern and Mountain states quickly pass these laws.

Now the anti-union impulse actually reaches a climax and a failure in 1958. You had the National Right to Work Committee, formed in 1955 by Southern textile and lumber interests. And in 1958 they put right-to-work laws up for a referendum in six states, including California and Ohio. And they failed. You saw a rebirth of liberalism that year.

BP: The right-to-work movement went dormant at that point?

NL: Not completely. At that point, the National Right to Work Committee shifted its strategy from trying to pass right-to-work laws in the states. Instead, it went to court to try to weaken existing union contracts, first in the private sector, then in the public sector.

It's interesting, when all those Democrats went to see [Michigan Gov. Rick] Snyder the other day, they told him that even under current rules, workers in unionized workplaces don't have to join the union, they just have to pay the costs of bargaining. Well, where did that come from? It was due to a legal victory by the National Right to Work Committee, the Beck case [in 1988]. The Supreme Court agreed — nonunion workers don't have to pay dues, they just have to pay fees to cover bargaining costs. It weakened the soul of the union, which is solidarity.

BP: The argument against these laws seems to be: If workers in a unionized workplace can opt out of a union — if they don't even have to pay fees for the union that's representing them in negotiations — then solidarity vanishes and the union is weakened.

NL: Go all the way back to the 19th century, from 1877 onward, when there were strikes and violence. Why was there violence? Because the only way unions could work is if they deprived the employers of labor. Workers set up picket lines, the government would send in the police or militias or the army. And the whole principle of solidarity was that the union had to be united to succeed.

Solidarity isn't a purely altruistic concept. Unions have to be a combat organization, ready to fight the boss. That means there is an element of coercion involved. It's like taxes. The price of civilization is taxes. The price of unionism is solidarity. And, yes, that does involve coercing people to contribute to the union. Unions are not like the NRA or the Sierra Club, they're not purely voluntary organizations. They were given a slice of state authority in order to solve the problem of industrial violence.

BP: Except that most unions don't even strike nowadays, correct?

NL: Right, the strike is almost non-existent today. It's hard to argue that unions today need solidarity to make sure the factory stays shut down. But what unions do do is politics. They need money, staff. They're the ones hustling for votes. That's where the battlefield is being fought. And the money to do that comes from dues. When you don't have that, unions shrink.

BP: Back to the history. The National Right to Work Committee goes back to the courts, winning victories there. But lately we've seen Republican politicians renew the battle against unions at the state level. Indiana, Wisconsin, Ohio, now Michigan.

NL: With the decline of union numbers and the rise of a certain type of libertarianism, conservatives have returned to their old strategy, especially in the last two or three years.

And here's the devilish logic of this right-to-work strategy. When times are tough, the union can't win many concessions. So then the union shop gets eliminated. And a lot of workers say, why should I keep paying my dues? And that weakens unions even further.

BP: It's a downward spiral. Now, the fact that a right-to-work law just passed in Michigan, home of the United Autoworkers, isn't that symbolically significant?

NL: Yes, though the UAW, which had 1.5 million workers in 1975, is now down to just 400,000 — and only about 200,000 are autoworkers. It was already shrinking, thanks to transplants, China, automation. These days, the state with the highest union density is actually New York, with its hospitals and the public sector. Even in Michigan many of the unions are now in the public sector.

BP: Do these right-to-work laws ever get repealed?

NL: Indiana did. It had a right-to-work law in 1957, but it was so unpopular that it was reversed about eight years later. But there aren't many cases.

BP: And are there other state-level strategies to restrict unions?

NL: In California, there are various initiatives that get on the ballot every five years—it was Proposition 32 this year—that, one way or another, restrict how unions can spend money. Unions always mobilize and defeat these initiatives. But if you're a clever billionaire, it's a good strategy. Unions are spending all this money defeating these propositions, but not spending money on organizing or X or Y or Z.

BP: So unions don't really have a good response to this state-level opposition?

NL: Unions will have to defend themselves. But here's a question. There have been hundreds of thousands of people flocking to Madison, Wisconsin, or Lansing to fight over this. The meaning of solidarity's what's at stake here. It approaches the sacred. But we don't hear that. The language of union leaders is that they're defending the middle class. But it's more than that. Workers are defending a concept, a sentiment, and a fundamental moral value.

BP: One of the arguments in favor of right-to-work laws is that unions will just have to prove their worth to members. Don't some unions manage to do this? Nevada is a right-to-work state with some thriving unions.

NL: Yeah, the Culinary union in Las Vegas. If you're a vigorous union, that's true. A lot of ethnic-based unions still show strong solidarity. Some places that can be done.

Alternatively, there's the Our Walmart campaign. They know they're not going to get a union in Walmart, so they're trying to create that sense of solidarity outside of the Wagner Act and the [National Labor Relations Board]. But that's very difficult to do, because you need victories to sustain morale.

BP: And it doesn't look like very many national Democrats are willing to fight hard for unions anymore.

NL: No, and here I'd add a criticism of Obama. The Obama campaign made a strategic decision that it wanted to win in non-union North Carolina, in non-union Virginia. So Obama didn't fly into Madison, Wisconsin during the recall of [Gov. Scott] Walker. Obama's campaign was not a referendum of unionism.

And I'd add a larger meta-historical point. This is the experiment America’s now having. Obama has just won re-election. A liberal president is popular. But juxtapose that with what just happened in Michigan. So the question is, can you have a liberal, progressive America without unions? History says no. For 200 years the existence of the union movement has been wedded to the rise of democracy, to the rise of liberalism. We saw this here, in South Korea, in Spain, in Africa. But now America is moving toward an experiment with whether it can have liberalism without unions. I think the answer is no. But we'll see.

Interview has been lightly edited for length and clarity.

Related: What do "right-to-work" laws do to a state's economy?