Voters often support increased spending for public transportation, but the motivation behind this support is as hard to discern as it is substantively important. Why do voters support transit? We use a survey-framing experiment (n = 1200) to evaluate the persuasive effects of common arguments for public transit. We study how different arguments change not just people’s attitudes toward transit, but also their willingness to become politically active on its behalf. Our findings are twofold. First, arguments that transit will reduce congestion or improve environmental outcomes (specifically, mitigate climate change) make people more supportive of public transportation, while arguments about making travel more convenient, helping the poor, or satisfying other people’s desire for transit do not. Second, congestion-based arguments are self-undermining with respect to activism: they make people more likely to support transit, but less likely to become activists for it. Congestion-based arguments are persuasive because they remind people of wasted time, but in reminding people of time-scarcity they reduce people’s willingness to become involved. Overall, our results suggest that the common reasons people use transit are not powerful political arguments for supporting it, and that one powerful political argument for supporting it, in addition to being inaccurate, might undermine efforts to organize on its behalf.