Online social networks have emerged as a significant platform for political discourse. In this paper we investigate what affects the level of participation of users in the political discussion. Specifically, are users more likely to be active when they are surrounded by like-minded individuals, or, alternatively, when their environment is heterogeneous, and so their messages might be carried to people with differing views. To answer this question, we analyzed the activity of about 200K Twitter users who expressed explicit support for one of the candidates of the 2012 US presidential election. We quantified the level of political activity (PA) of users by the fraction of political tweets in their posts, and analyzed the relationship between PA and measures of the users' political environment. These measures were designed to assess the likemindedness, e.g., the fraction of users with similar political views, of their virtual and geographic environments. Our results showed that high PA is usually obtained by users in politically balanced virtual environment. This is in line with the disagreement theory of political science that states that a user's PA is invigorated by the disagreement with their peers. Our results also show that users surrounded by politically like-minded virtual peers tend to have low PA. This observation contradicts the echo chamber amplification theory that states that a person tends to be more politically active when surrounded by like-minded people. Finally, we observe that the likemindedness of the geographical environment does not affect PA. We thus conclude that PA of users is independent of the likemindedness of their geographical environment and is correlated with likemindedness of their virtual environment. The exact form of correlation manifests the phenomenon of disagreement and, in a majority of settings, contradicts the echo chamber amplification theory.