Recent scholarship has been reasonably optimistic about unionization as a mechanism of labour justice for immigrant workers in casual and contingent work. This optimism rests on two assumptions: (1) that unions have the capacity to absorb immigrant workers in nonstandard work and (2) that casual, immigrant labourers enjoy the kind of solidarity that underpins collective action. This paper examines these assumptions critically through a case study of construction unions and Latino immigrant day labourers in Denver, Colorado and Baltimore, Maryland. I use participant observation and in-depth interviews with nine labour unions, 19 Latino immigrant day labourers, and two (non-union) day labour organizing projects in the cities to examine questions of capacity and solidarity. I find that the existing foundations for unionizing day labourers may be weak in certain cities and communities. Union capacity is undermined by structural fragmentation and specialization in market segments that are inaccessible to day labourers. Strategically, in an age of de-unionization, unions also face pressures to “add value” for employers by sorting the workforce into high quality and low quality categories. Locals indicate day labourers would likely fall into the latter category, thus precluding membership. The foundations for solidarity are similarly weakened in the cases studied. Culturally, day labourers in Denver and Baltimore emphasize self-reliance and material well-being over collective action and the pursuit of justice. To work toward unionization, organizers should be prepared to confront deficits of capacity and solidarity in other cities as well, especially those where homelessness is prevalent among day labourers, where immigrant populations are newly arrived, or where local union cultures are unreformed. I suggest that union collaboration, a cooperative type of occupational unionism, and commitments to training day labourers may help boost union capacity to absorb day labourers, while the creative use of material incentives should figure prominently in organizing strategies.