Debates about climate change often involve ad hominem attacks. Each side is accused of insincerity, of merely serving special interests. In particular, those who advocate policies to promote energy conservation or otherwise reduce CO2 emissions can be challenged if their personal energy use appears to be high. Our studies indicate that an attack based on high personal carbon footprint can be extremely damaging. In our first two surveys, participants read vignettes in which a climate researcher advocates personal energy conservation. Participants were randomly assigned to different vignettes, in which the researcher’s own behavior either matched his message or fell short (in various ways). Falling short vastly reduces the researcher’s rated credibility; moreover, these survey participants report much weaker intentions to conserve energy. However, if the vignette indicated a favorable change in researcher behavior, credibility was restored and participants’ intentions strengthened. In a third survey we focused on climate policies aimed to reduce CO2 emissions rather than on voluntary conservation. Again, the researcher’s credibility is reduced, along with support for the policy he advocates, when his personal behavior does not match the energy conservation message. While participants’ beliefs about climate change and their political orientation also strongly affect ratings of researcher credibility, intentions to conserve energy, and support for policies to reduce CO2 emissions, the ad hominem effect of the researcher’s personal behavior remains strong for all participant subgroups. Those who advocate change need to prepare for ad hominem attacks by examining and changing their own behavior.