Candidates face a trade-off in the general election between taking a more-moderate position that appeals to swing voters and a more-extreme position that appeals to voters in the party's base. The threat of abstention by voters in the party's base if their candidate takes a position too moderate for them moves candidates to take more-extreme positions. I discuss hypotheses regarding how this trade-off affects candidate positioning and describe my tests of those hypotheses using data on House members in the 107th Congress and Senate members for the period 1982–2004. I then present data on how the distribution of voters in the electorate has changed over the past three decades and discuss how, in light of my empirical findings, these changes might explain the observed pattern of asymmetric polarization in Congress in recent decades.