In this essay, I contend that one can understand neither the development of mass action among contemporary immigrants, nor the sporadic nature of that action, without attending to the historic role of parties and community-based organizations in shaping immigrants' political mobilization. I draw connections between the mass immigrant-rights demonstrations that took place during the spring of 2006 and what we know about how immigrants' political participation in the United States is structured by (1) the declining influence of political parties, and (2) the critical function of community-based organizations. These organizations were the focus of my recent book, Democracy's Promise: Immigrants and American Civic Institutions (2006). Why haven't activists been able to sustain the momentum that brought hundreds of thousands of immigrants and their supporters out into the streets during the spring of 2006? Although they, along with the Spanish-language media, played a critical role in organizing mass demonstrations against punitive immigration legislation in early 2006, labor organizations, workers' centers, advocacy and social service organizations, ethnic voluntary associations, and religious institutions face severe constraints in terms of engaging in sustained, consistent political mobilization and, therefore, mainly achieve limited mobilization. However, voter registration data from the National Association of Latino Elected Officials suggest that the demonstrations may have spurred interest in more traditional types of political participation among immigrants and their supporters. Thus, while it is true that, for the most part, political participation does not take place overnight, there may be ways for U.S. civic institutions to speed up that process through direct mobilization and the provision of information that helps immigrants to feel more comfortable and confident taking part in the political system. Trusted community-based institutions represent a vital potential force in promoting political inclusion for immigrant newcomers who contribute to so many other aspects of American life.