The structure of online political discussion has proven important to deliberative democracy. However, the organizational mechanisms of the structure receive little attention in scholarship. This study employed a random-effects relational event model to differentiate and examine the effects of a set of organizational principles in web forum discussions. By analyzing more than 175,000 forum replies, the study found that cross-ideological debate is an independent organizational mechanism even when accounting for the effects from common interests, opinion congruity, purely structural effects, and conversational norms. These findings differ from the selective exposure thesis and previous incidental claims of political disagreement. In addition, the findings indicate that endogenous mechanisms and opinion congruity could influence the tendency of cross-ideological debate to varying degrees.