John gives some reasons why viewers of the political scene might think that congressional Republicans are more disciplined than their Democratic counterparts, even if this isn’t really so.
I'd like to give one more big reason based on recent history. When Barack Obama became president, congressional Republicans implemented a solid No strategy and were successful at stopping much of the Democrats' agenda. With only 40% of each of the houses of Congress, the Republicans had few tools except party discipline. To me, it is this impressive achievement that earns congressional Republicans their reputation.
Everything John wrote is fine but I think he's missing the elephant in the room (so to speak).
