How Congress became so partisan, in 4 charts

As we've noted before, the ideological middle is dead in Congress.

With Capitol Hill quiet as members of Congress travel home for a two-week recess, what better time to dive a little deeper into how, exactly, we got to the point where we are today -- the legislative branch being more divided than ever before in American history.

Late last year, the Brookings Institute published an interactive graphic showing how the partisan divide between Democrats and Republicans has varied over time, charting out the relative conservatism and liberalism of each Congress.

For example, the 112th Congress (last year) looks like this:

Meanwhile, members of the parties voted almost exclusively along party lines.

But it hasn't always been this way. Lets go back in time a decade to the 107th Congress.

Two Democrats (Ralph Hall of Texas and James Traficant of Ohio) were more conservative than Connie Morella of Maryland, who was the most liberal Republican member of Congress.

Still, those three remained complete outliers, with the caucuses voting almost exclusively along party lines and with the Democrats and Republicans still far apart on the ideological spectrum.

So, when did things get so polarized? The 1990s.

Lets jump backward one more decade.

Republicans like Morella and Frank Horton (New York) voted consistently more liberal than Democrats like Ralph Hall and Billy Tauzin (La.). The ideological divide remained this way for much of the 1990s, although the parties slowly became more and more partisan.

Now, lets jump even further back in time -- to the 91st Congress, in office from 1969 to 1970.

Several Democrats were drastically more conservative than several Republicans, and there were a number of Republicans who consistently fell on the liberal side of the spectrum.

Wanna see more? Make sure to head over to Brookings, which graphs out every Congress going back to 1857.