A lot of press coverage has focused on the relatively small group of lawmakers, many of them tea party endorsed backbenchers, who pushed their leadership into shutting down the government in a desperate last-gasp effort to kill off Obamacare.

But how does that work? Why can a cabal of hard-right legislators tell their party’s leadership what to do? Why doesn’t the rest of the Republican caucus stop them? The answer is that in the wake of the Supreme Court’s Citizens United decision a well-funded network of conservative groups has arisen that has the ability to make the lives of Republicans who seek compromise very, very difficult.

Legislators are hesitant to cross these enforcers, fearing primary challengers from the right, or grassroots tea party activists flooding their offices with angry calls. Here’s a look at some of the key players behind the scenes (above) and some of the key lawmakers in front of the cameras (below). Roll over the heads to learn more about each player.