The number of website visits, Facebook friends, or Twitter followers that politicians attract varies greatly, but little is known about what drives politicians' online popularity. In this article, we use data from a systematic tracking of congressional candidates' popularity on four web platforms in the 112 most competitive congressional districts in the 2010 U.S. midterm elections to address that question. Using multivariate regression models, we show that while district-level socioeconomic characteristics have little effect on candidates' online popularity, challengers and candidates in open-seat races tend to attract larger audiences online, as do candidates who are more visible on political blogs. Surprisingly, how intensely candidates are covered in news media, how popular they are in opinion polls, and how much money they spend during the campaign show no significant effect. These findings help us understand the dynamics of internet politics, and have wider implications for candidate competition and party politics.