Why have the people running for Congress become more ideologically extreme over time, causing Congress to polarize? In this paper, we study how the pool of state legislators shapes the ideological distribution of congressional candidates. We geographically match state legislators to the US House districts that they could plausibly seek to represent. Although the national pool of state legislators has polarized over time, we find no evidence that they have polarized more where opportunities to win House seats are higher, or that more-extreme state legislators have become more likely to run for Congress relative to other state legislators in the same congressional districts. We conclude that the nationwide polarization of state legislators is particularly important in explaining the polarization of congressional candidates. The results emphasize the need to understand ideological selection into running for lower-level political offices in order to understand congressional polarization.