Both parties threaten their junior members to tow the party line or they will work against them next election.
Actually, that's not what is happening. It's just a simple matter of the congressmen not needing to give a shit about the voters.
How can that be so in a democracy? Simple: in the US, you allow the politicians to draw the borders between electoral districts. So they redraw the borders to ensure they have a safe seat. Once you have a safe seat, there is no need to care about the people who voted you in.
the two dominant parties in the state of California cooperatively redrew both state and Federal legislative districts to preserve the status quo, ensuring the electoral safety of the politicians from unpredictable voting by the electorate. This move proved completely effective, as no State or Federal legislative office changed party in the 2004 election, although 53 congressional, 20 state senate, and 80 state assembly seats were potentially at risk.
Most western countries have cured themselves of this particular curse, but not the USA. Again quoting Wikipedia:
Due to the perceived issues associated with gerrymandering and its impact on competitive elections and democratic accountability, numerous countries have enacted reforms making the practice either more difficult or less effective. Countries such as the U.K., Australia, Canada and most of those in Europe have transferred responsibility for defining constituency boundaries to neutral or cross-party bodies.
So you might ask yourself, if a typical congressman doesn't have to care about what he voters think, what does he care about? Enriching himself of course. How does he do that? By passing laws allowing anybody to throw unlimited amounts of cash in his direction in return for dispensing favours.
And when that plays out, what do you end up with? Laws that favour the rich of course. And what does a country look like when it allows the rich to run the place? Like the USA of course, where 1% of the people control 35% of the wealth, and 40% people control 0.2%.
You yankies never cease to amaze me.