Why does the influence of Congressional parties fluctuate over time? Building on prevailing answers, we develop a model, Strategic Party Government, which highlights the electoral motives of legislative parties and the strategic interaction between parties. We test this theory using the entire range of House and Senate party behavior from 1789 to 2000 and find that the strategic behavior of parties complements members' preferences as an explanation for variation in party influence. Specifically, the strongest predictors of one party's voting unity are the unity of the opposing party and the difference between the parties in the preceding year. Moreover, we find strong links between party behavior in Congress and electoral outcomes: an increase in partisan influence on legislative voting has adverse electoral costs, while winning contested votes has electoral benefits.