Studies of American politics, particularly legislative politics, rely heavily on measures of the partisanship of a district. We develop a measurement model for this concept, estimating partisanship in the absence of election-specific, short-term factors, such as national-level swings specific to particular elections, incumbency advantage, and home-state effects in presidential elections. We estimate the measurement model using electoral returns and district-level demographic characteristics spanning five decades (1952–2000), letting us assess how the distribution of district partisanship has changed over time, in response to population movements and redistricting, particularly via the creation of majority-minority districts. We validate the partisanship measure with an analysis of congressional roll-call data. The model is easily extended to incorporate other indicators of district partisanship, such as survey data.