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:

As you can see from the chart, not a single Democratic member of the 112th Congress was more conservative than a single Republican member, and not a single Republican member was more liberal than a single Democrat.

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.

As you can see, the parties were still largely politically polarized, but 2001-2002 was the last time that some members of Congress were consistently voting out-of-ideological-sync with their parties.

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.

While the hyper-partisans of each party remained far apart on the ideological map, the early 90s were a time when each party had several members who were relatively out of step with the rest of their caucus comrades.

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.

The 91st Congress was -- easily -- the year when the parties were the most ideologically diverse.

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.