Barry Eidlin is an assistant professor of sociology at McGill University, and the author of "Labor and the Class Idea in the United States and Canada."

Verizon workers picket outside one of the company’s facilities in Philadelphia in April. (Matt Rourke/Associated Press)

Last week we published a series diving into the question of why unions are so weak in the United States. Below is a response to that series.

Barry Eidlin is an assistant professor of sociology at McGill University. His forthcoming book, “Labor and the Class Idea in the United States and Canada,” explains why union strength diverged in the United States and Canada over the course of the 20th century. Find him on Twitter: @eidlin.

U.S. unions haven’t declined because they’re unpopular. Today, a majority of Americans approve of unions, and far more U.S. workers would like to join unions than are able to.

Nor is the decline of unions due to the loss of unionized manufacturing jobs. Plenty of other countries have shifted to a service economy without as much of a fall in membership. Labor laws — another common explanation for union weakness — are certainly stacked against U.S. workers. But this is more a symptom than a cause.

This becomes clearer when we look at Canada. Unions there are currently much stronger than their U.S. counterparts, as is Canadian labor law. But that wasn’t always the case. Up until the mid-1960s, union density was similar in both countries, but then diverged. While some argue that Canada’s stronger labor laws explain the shift, many of the key legal reforms in Canada took place a decade or more after union density began to diverge. Something else was at work. My research indicates that it involves something that is once again a topic of conversation this election season: the role of left-leaning third parties.

From a paper I wrote last year in Politics & Society:

Fundamentally, U.S. unions have lacked the legitimacy they retain in other industrialized countries. Even though unions have helped to reduce overall income inequality and to pass reforms that have benefited all workers, they are painted as parasitic “special interests” that gain only at the public’s expense.

Why is that? More than anything, it’s because the United States has historically lacked a strong, independent, political left. In other countries, workers organizing to defend their rights not only formed unions to protect them on the job, but also labor or socialist political parties to protect them as a class — the working class. Building off surges of worker protest, these parties won pro-labor reforms, either by winning office or posing enough of a political threat to get ruling parties to act. More broadly, they expanded notions of democratic citizenship to include many of the social welfare benefits like health care and old age security that are now taken for granted. Overall, this wove workers’ rights more tightly into the fabric of democracy, making it harder to unravel them.

This didn’t happen in the United States. More precisely, it didn’t happen in the same way. American workers fought for labor rights for decades, in some cases tying their workplace struggles to broader political movements and parties. Despite the many barriers to third parties in the U.S., these parties managed to capture a small but significant part of the vote in the late 19th and early 20th centuries, even holding some state-level offices.

But that changed in the 1930s. Although he came into office as a budget-cutting deficit hawk, President Franklin Roosevelt’s advisors convinced him that responding to growing worker and farmer protest with reforms could bring these groups into the Democratic Party coalition. FDR’s rhetorical appeals to the “forgotten man” and policy offerings like the National Labor Relations Act absorbed key parts of these protest groups while dividing and excluding others. On the one hand, this consolidated the liberal coalition that characterizes the Democratic Party to this day. On the other, it decisively undermined any left alternative to the Democrats.

Again, looking at Canada is instructive. Despite fewer barriers to third parties there, they had limited success until the 1930s. At that point, both the mainstream parties, the Liberals and Conservatives, responded to worker and farmer protests not with reforms, but with repression. This drove the excluded groups to form an independent party, the Cooperative Commonwealth Federation, which took root and lives on today as the New Democratic Party.

From my research in American Sociological Review:

U.S. unions seemed to get the better deal at the time, but the New Deal coalition was ultimately a “barren marriage. As a junior partner within the Democratic Party, labor focused on its “inside game” of influencing sympathetic allies to win reforms. Whatever bargains it could win thus appeared not as broad gains for workers, but as payoffs to a narrow Democrat “special interest.” By contrast, Canadian labor’s electoral threat combined with worker mobilization created a bargaining process to enforce industrial peace, one that even labor’s opponents understood the value of maintaining. This ensured a more legitimate Canadian labor law regime that strengthened over time.

Meanwhile, U.S. labor began to atrophy, even as it reached new heights of influence. Conservative elements within labor used the Taft-Hartley Act to dismantle Communist-led unions, in the process expelling some of its best organizers and diluting its mobilizing capacity. As for labor law, it became less about promoting industrial peace and more about “balancing” workers’ collective bargaining rights against ever-expanding individual and employer free speech and property rights. This restricted unions’ ability to protest and paved the way for right-to-work laws that undermined union solidarity. The result was union decline.

Where does this leave us today? There are no easy answers. Recent actions such as the Fight for $15 campaign and the Verizon strike hint at the revival of more disruptive mobilizing strategies, while the improbable rise of the Bernie Sanders campaign suggests that there is room for a more robust class politics in the U.S. political arena. Yet labor’s lukewarm response to Sanders and his ultimate failure to secure the Democratic Party nomination show how much unions remain ensnared in their “special interest” straitjacket. The ties may be loosening, but breaking out will be a challenge.

Read more:

Kimberly Phillips-Fein: How employers broke unions by creating a culture of fear

Michael Wasser: Unions could make a comeback — if we help them

Cathie Jo Martin: Business and labor don’t have to be enemies

Joseph A. McCartin: This election could decide the fate of American labor — and that’s the problem