Political parties are often faced with seemingly opposing goals when trying to secure members’ reelection and maintain party unity. On one hand, a party needs to fulfill members’ diverse electoral needs for their reelection, and on the other hand, the party must force members to vote in unison according to party lines for collective decisions. How does a party influence its members to take unified action while meeting their individual electoral needs? Through an analysis of the Japanese Diet, this study argues that parties attempt to achieve the reelection of their members and maintain party unity by manipulating legislative committee assignments and deliberations. In particular, the study demonstrates that a party shapes committees in a different way according to policy areas over which committees have jurisdiction. A party tends to accept its members’ requests for affiliation and allow their self-management in committees concerned with particularistic benefits so that they can deliver specific benefits to each electoral district. In addition, a party tends to assign members who have average policy positions in the party to committees concerned with general benefits to make policies that satisfy many constituencies.