Since 1972, campaign spending by House incumbents has skyrocketed, particularly in those districts with marginal support for the incumbent's party. At the same time, parties in the House have become much more cohesive in the way they vote, producing more precise and informative party brands. We argue that these two phenomena are fundamentally linked. As parties have developed more precise reputations, incumbents in these districts must spend much more to attract voters in “marginal” districts, who would be willing to vote for a candidate with the particular incumbent's legislative record, but not the average member of his party. Increasingly precise party reputations provide voters with stronger priors that incumbents are just like the rest of their party, and incumbents in marginal districts must spend more to overcome these beliefs. We demonstrate this using a simple formal model and test it empirically using campaign-spending data from 1972 to 2008.