Mobilizing citizen climate lobbying among Republicans may be a key tool in building political will for bipartisan climate policy in Congress. However, record-high issue polarization— partisan gaps in beliefs—and affective polarization—animosity towards counter-partisans— may hold back efforts to expand the left-leaning citizen climate movement across party lines. We run a series of online experiments with 20,000 participants testing how polarization shapes the spread of citizen movements. When we randomly pair Democrats with Americans across the political spectrum, they are 27% more likely to invite other Democrats than Republicans to email Congress about climate change, even when all of them believe climate change is human caused. We find three explanations for this outreach gap. First, Democrats correctly believe that their invitation will have half as much impact on Republicans’ action. Second, these strategic beliefs are driven by anticipated affective polarization, or the idea that Republicans will react badly to outreach from Democrats. Finally, Democrats’ own affective polarization matters: they prefer to reach out to co-partisans even when cross-party outreach is as effective.