There is, really, only one thing you need to understand if you want to see why Congress doesn't do much of anything these days. And that one thing is this: We are living in a time of historic polarization between the two parties.

The new Vital Statistics on Congress, which is amazing and filled with great stuff, presents this fact in stark terms.

First, let's look at the ideological positioning of the two parties in the House over the past 60 years or so. (For a more detailed description of how Vital Stats developed these ideological ratings, check this out.)

As you can see, partisanship of both Republicans and Democrats in the House has been on the rise since, roughly, the election of President Bill Clinton in the early 1990s.

But, the partisanship hasn't grown equally on both sides.  While Democrats have grown gradually more partisan over the past two decades, it has been a gentle slope toward more liberal voting.  Republicans' move toward a more conservative position has been far more steep and, as National Journal's Niraj Chokshi pointed out today, the House GOP is now at the most conservative point it has been in six decades.

There's lots and lots of reasons for this. Here are a few.

* Redistricting over the past two decades has made the vast majority of House districts safe for one party or the other.  That means that elected officials -- in both parties -- are politically incentivized to retreat to their bases and not worry about the middle.

* Since the 1994 Republican revolution, the rallying cry within the GOP has been that the only or at least best path to electoral success is a focus on conservative principles. The rise of the tea party in 2009 and, particularly, following the 2010 election led to a re-affirmation of the need to stay true to conservatism's founding principles.  And the defeats of people like Sens. Dick Lugar (R-Ind.) and Bob Bennett (R-Utah) in intraparty skirmishes has made clear the political danger of being labeled as a centrist -- or anything close to it -- in the GOP.

Regardless of the reasons, the chart above makes clear that the two parties in the House have, literally, never been further apart ideologically.  They see the political world through totally different lenses/glasses/eyewear metaphor of your choice. It's why --from issues simple to complex -- Congress can't get anything done.  And they will continue to not get anything done, at least for the foreseeable future.