Interviewer effects in telephone surveys on political topics are likely to occur. The literature has yielded considerable evidence about the impact of basic interviewer characteristics, but research is lacking on how interviewers' beliefs may shape responses. This study is aimed at assessing the association between the interviewers' party affiliation and political ideology and the answers provided to the same questions by the respondents, using a large-scale telephone survey conducted in Hungary. The results show that interviewers account for a relatively large amount of variance, especially in the models predicting item-nonresponse. The interviewer's reluctance to answer these questions strongly predicted missing data on the respondent level. On the other hand, only weak interviewer effects were found regarding the valid, substantive responses. The results demonstrate how, for instance, estimations of nonrespondents' party preferences in electoral polls may be biased due to interviewer-related errors, and highlight the need for standardized persuasion strategies.