The House Rules Committee. People usually call this one of the most important House committees out there, which is in my opinion untrue: it simply is the most important House committee. The reason that I say that is because the Rules committee has ultimate control over how and in what way a bill is presented and debated; add that to its ability to dictate appropriate amendments leads to an effective result of Rules being the gatekeeper for House legislation. The membership is deliberately skewed heavily in favor of the majority party (currently over two-to-one), and majority party membership on that Committee is at the discretion of the Speaker of the House. In other words, if a Member of Congress disapproves of the way that the Rules Committee operates, the only way to show disapproval is to vote for somebody else for Speaker of the House.
The Democratic vote to re-elect Nancy Pelosi as Speaker of the House for the 111th Congress was 255 For, 1 Not Voting. Every action - and every flawed piece of legislation - that made it past the Rules Committee since then is thus the responsibility of those 255 Members of Congress who authorized giving control of the American legislative agenda to Speaker Pelosi.
And that is why there is no such thing as a "conservative" Democratic politician. That first vote defines all the rest.
Crossposted to Moe Lane.