Politicians’ dual responsibilities to respect their party and also be responsive to their constituents is surprisingly lacking in studies of representation. How do politicians—especially those who function in strong-party systems—individually respond to their constituents’ preferences? We make use of an original, large-scale survey of politicians and the recent success of the Sweden Democrats in the elections in Sweden to show that important adaptation takes place within the party structure. Individual politicians are responsive to signals about voters’ preferences, and they act on these signals by internally lobbying their party leaders to change the party’s positions in the direction of their constituents’ preferences. These results provide a rationale for why niche parties invest in elections even if they are unlikely to enter government: Their electoral successes can cause change in other parties. The results also add a new angle to the discussion of how anti-immigration parties affect mainstream parties, a hotly debated issue in many advanced democracies.