Capital & Main is an award-winning publication that reports from California on economic, political and social issues. The American Prospect is co-publishing this piece.

Mark Janus, an Illinois child-support worker, will soon argue before the U.S. Supreme Court that his free-speech rights have been violated because he must pay “agency fees” to a union that negotiates contracts on his behalf. Last year, California elementary school teacher Rebecca Friedrichs made the same First Amendment arguments at the high court against the teachers association to which she paid agency fees. The court deadlocked on Friedrichs's complaint following the death of conservative Justice Antonin Scalia, but his replacement, Justice Neil Gorsuch, is widely expected to cast a decisive vote in favor of Janus and against public-sector unions in Janus v. American Federation of State, County, and Municipal Employees.

In 2012, as Election Day neared, polls showed dwindling support for California's Proposition 30, which sought to avoid a crippling $6 billion cut in the state's public education system. Without Prop 30, Governor Jerry Brown warned, many more teachers would be laid off from a system already racked by cutbacks, and the schoolyear would likely be slashed by three weeks.

Despite a $36 million donation from Republican activist Charles Munger Jr. to a committee opposing both Prop 30 and another ballot measure, along with a competing proposition sponsored with $44 million from his sister, Molly Munger, voters passed the initiative by a 55-to-46 margin. By doing so, they rescued the school system by reinvesting billions of dollars into preschool, K-12, and community colleges. And voters did so, in large part, thanks to the political muscle of the state's public-sector unions.

In the weeks leading up to the election, armies of union members fanned out across the state to knock on doors, while three unions—the California Teachers Association, the statewide council of the Service Employees International Union, and the American Federation of Teachers—spent a combined $26 million in support of the initiative, serving as a counterbalance to the deep pockets of the Mungers and other wealthy donors. Last year, as Prop 30 was set to expire, unions pushed through Proposition 55, which extended the income tax on high earners for another dozen years.

“The truth is, in the last 25 years, no major progressive proposition has passed without labor being a primary donor,” said Kenneth Burt, who retired last month after spending two decades as the political director of the California Federation of Teachers (CFT). A 2012 study of public-sector union influence over California ballot initiatives since 1980, prepared by the conservative Manhattan Institute, came to the same conclusion. “Whenever a proposal was especially important to the unions,” its author wrote, “they almost always won.”

California's 1.4 million–member public-sector unions are the key force that has pushed the state toward increasingly progressive policies. They have set California on the path to a $15 hourly minimum wage, helped pass a “Sanctuary State” bill to protect undocumented immigrants, supported a $4 billion affordable housing bond on next year's ballot, and spurred legislators to contemplate universal health-care legislation.

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But what would happen to such legislation if, as expected, the Supreme Court rules against them in the Janus case? Would it even be proposed?

In ballot-measure fights, the opposition to workplace, environmental, and health-care reform is typically well-funded, and if union membership and dues were to shrink by, say, one-third, they could suddenly find themselves outgunned.

“We are concerned about attacks on the labor movement, because they would weaken the strength of patient and community voices,” said Anthony Wright, the executive director of Health Access, a statewide health-care advocacy coalition. Wright noted that labor union support had been critical in efforts to expand coverage in California, and pointed out two recent legislative achievements in which labor played a key role. Last year, Assembly Bill 72, which prevents patients from being hit with surprise medical mills, was passed. And last month, Governor Brown signed Senate Bill 17, which makes prescription drug prices more transparent. (Drug companies spent $16.8 million in opposition to SB 17.)

In 2010, the CFT's Kenneth Burt chaired the group that sought to pass Proposition 25, which allowed the state legislature to pass the budget by a majority vote, replacing the earlier two-thirds requirement.

“That two-thirds requirement meant that even during flush years we had to make concessions to the Republicans—and those concessions were tax breaks to the rich and cuts to social programs,” said Burt. The forces against the measure included Chevron, Philip Morris, and the California Chamber of Commerce, which spent a combined $17.7 million. The union-led coalition invested nearly as much, $15.2 million, and they won. But if California's public-sector unions shrink, such fights could become lopsided defeats.

Nelson Lichtenstein, a labor historian at the University of California, Santa Barbara, cautions that one shouldn't think of the union's power—in California and elsewhere—as primarily being its lobbying ability or financial resources. “The key thing about unions is not the money,” Lichtenstein said, "but the energy that comes out of the members. Having a lot of cash is always second-best.”

“The reason that conservatives hate unions is not because they are winning huge raises all the time,” he added. “It's because day in and day out, they are always there. Activist movements get a lot of headlines, but the union is an institution. This is trench warfare—you win by a few inches here and there. Reducing union dues will hurt that.”