Why are class politics more prevalent in Canada than in the U.S., even though the two countries share similar cultures, societies, and economies? Many view this crossborder distinction as a byproduct of longstanding differences in political cultures and institutions, but I find that it is actually a relatively recent divergence resulting from how the working class was politically incorporated in both countries before, during, and after World War II. My central argument is that in Canada, this incorporation process embedded "the class idea"--The idea of class as a salient, legitimate political category--more deeply in policies, institutions, and practices than in the U.S. Out of the social and political struggles of that period emerged two working class movements that, although bearing a surface resemblance, were organized along different logics. In Canada, the working class was incorporated as a class representative, whereas in the U.S. It was incorporated as an interest group. That difference in political incorporation enabled or constrained labor's legitimacy and organizational capacity in different ways in both countries. Canadian labor's role as a class representative legitimized it and expanded its organizational capacity, while U.S. labor's role as an interest group delegitimized it and undermined its organizational capacity. I show this through a detailed analysis of trajectories of labor movement strength in both countries over the course of the twentieth century, as measured by unionization rates, or union density. Starting from the observation that union density was very similar in both countries until the mid-1960s, then diverged, I first examine competing explanations for this divergence. Having illustrated their strengths and limitations, I then develop an argument showing how the divergence in working class organizational strength was the outcome of struggles for political incorporation. I identify two key moments that shaped these different processes of political incorporation. The first was the restructuring of party-class alliances in both countries in the 1930s and 40s, where U.S. labor decisively abandoned the project of building an independent working class party in favor of an alliance with the Democratic Party, at the same moment that Canadian labor forged an independent class alliance with progressive agrarian forces under the banner of the CCF. The second was differences in the effects of postwar Red scares on the relationship between labor and the left in both countries. While anti-Communism took its toll on working class movements in both countries, the labor-left alliance was severed in the U.S., but only strained in Canada. The outcome of these processes was a U.S. labor movement that conceived of itself more as an interest group representing a specific constituency within the Democratic Party, and a Canadian labor movement that conceived of itself more as a class representative with closer ties to a broader social movement. Differences in labor's political incorporation also shaped the formation and development of the regimes governing labor-management relations in both countries. The Canadian labor regime was created as a result of working class upsurge from below, whereas the U.S. labor regime was created as part of an elite reform project from above. This original difference influenced the organizing logics of each regime. Whereas the Canadian labor regime was organized around recognizing the existence of class conflict and seeking to mitigate it, the U.S. regime was organized around protecting workers' individual rights. Although this created a more interventionist Canadian system that restricted labor's scope of action in important ways, it also reinforced a collective, oppositional class identity vis-à-vis both employers and the state. Meanwhile, the U.S. system's focus on rights led to a stronger focus on legalistic proceduralism and imposing a formal equality between labor and management that obscured the power imbalance inherent in the employment relationship. Additionally, labor drew different lessons from these different processes of regime formation. Whereas Canadian labor learned the value of winning gains through disruptive mass mobilization, U.S. labor learned the value of winning gains through sympathetic politicians and favorable legal precedents. The combination of a more protective and institutionally stable labor regime and a labor movement more accustomed to winning gains through mass mobilization, Canadian labor was better positioned to defend itself than its U.S. counterpart when employers began a counter-offensive beginning in the late 1960s. While U.S. labor spiraled into decline, Canadian labor proved more resilient, leading to the divergence in union density rates.