Does enumerating undocumented residents in the U.S. census affect congressional apportionment?

Abstract In recent years American political figures, media pundits, and others have argued that undocumented residents in the United States should not be included in census data used for congressional apportionment. They argue that including them unfairly benefits some states ─ especially politically Democratic leaning (or “blue”) states, according to most arguments ─ at the expense of others. Some people have speculated that many more House seats and Electoral College votes would have been apportioned to politically Republican leaning (or “red”) states and that many fewer would have been apportioned to politically “blue” states if undocumented residents were excluded from apportionment data. The only systematic empirical examinations of this question occurred prior to the 2020 Census and projected ─ mostly inaccurately ─ how apportionment would differ after the 2020 Census under this hypothetical. Does including undocumented residents in U.S. census data used for congressional apportionment alter (a) political party representation in the House of Representatives or (b) presidential electoral college votes? We use annual state-level estimates of the undocumented resident population at each census date to examine the consequences of including them in official apportionment data since 1980. If undocumented residents had been entirely excluded from census data used for apportionment, no more than two House seats and three electoral college votes would have shifted between political parties in any year since 1980; this would have had no bearing on party control of the House or the outcome of presidential elections.