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Almost all legislators are subordinate to party leadership within their assemblies. Institutional factors shape whether, and to what degree, legislators are also subject to pressure from other principals whose demands may conflict with those of party leaders. This article presents a set of hypotheses on the nature of competing pressures driven by formal political institutions and tests the hypotheses against a new dataset of legislative votes from across 19 different countries. Voting unity is lower where legislators are elected under rules that provide for intraparty competition than where party lists are closed, marginally lower in federal than unitary systems, and the effects on party unity of being in government differ in parliamentary versus presidential systems. In the former, governing parties are more unified than the opposition, win more, and suffer fewer losses due to disunity. In systems with elected presidents, governing parties experience no such boosts in floor unity, and their legislative losses are more apt to result from cross-voting.