We find that parties with unified control in Washington since the Clinton years have struggled for two reasons.

The filibuster explains some of the majority parties’ struggles. Senate rules require most legislation to obtain 60 votes to advance to passage. As a result, minority parties have a chance to either veto or reshape most legislation. Still, even though it’s a constant source of discussion and debate in today’s Washington, we find the filibuster was the cause of only one-third of failed attempts by majority parties to enact their priorities during unified government since 1993.

The second reason is less well appreciated but accounts for the other two-thirds — a large majority — of failures. Both parties have been, and remain, internally divided on many issues. Parties are often able to hide their disagreements by simply not taking up legislation on issues that evoke significant fissures. But when those issues reflect their campaign promises, majority parties will often forge ahead even in the absence of internal consensus on a plan.

Whether Democratic or Republican, the party with unified control in Washington in recent years has failed on one or more of its highest-priority agenda items because of insufficient unity within its own ranks.