Campaigns, parties, interest groups, pollsters, and political scientists rely on voter-registration lists and consumer files to identify people as targets for registration drives, persuasion, and mobilization and to be included in sampling frames for surveys. We introduce a new category of Americans: the politically invisible— that is, people who are unreachable using these voter and marketing lists. Matching a high-quality, random sample of the US population to multiple lists reveals that at least 11% of the adult citizenry is unlisted. An additional 12% is mislisted (i.e., not living at their recorded address). These groups are invisible to list-based campaigns and research, making them difficult or impossible to contact. Two in five Blacks and (citizen) Hispanics are unreachable, but only 18% of whites. The unreachable are poorer than the reachable population, have markedly lower levels of political engagement, and are much less likely to report contact with candidates and campaigns. They are heavily Democratic in party identification and vote intention, favoring Obama versus Romney 73 to 27, with only 16% identifying as Republicans. That the politically invisible are more liberal and from historically marginalized groups shows that the turn to list-based campaigning and research could worsen existing biases in the political system.