Party Loyalty
Democrats & Liberals:: Heckling Pelosi
Crowley tells us that Democrats with different points of view have been elected. So what? Regardless of what their specific views on specific issues are, they are all to the left of the Republicans still in power. This gives Pelosi a great tool for unification of the party. Some Democrats may digress occasionally from the party line, but it will be to their advantage to not do it often.
And here is one of the major problems with the 2 party system, members are expected to be loyal to their party over their loyalty to their constituents. If there were, say, 10 parties, it would be easy for someone to stay loyal to something other than their party, as they could just find another party if their own started doing something stupid.