Allow us to tap out the now-standard drumbeat: Capitol Hill was more polarized in 2014 than it was in 2013, which was more polarized than it was in 2012, and so on until you get to the mid-1970s or so. We've heard this before. But: Why did this happen?

First, the evidence. Using a metric called DW-NOMINATE compiled by VoteView -- a site maintained by political scientists at the University of Georgia -- we can see the divergence between average polarization scores for each party in the House over time. (If you're curious about the components of the score, you can read this.)

Our colleagues at Wonkblog isolated a particularly remarkable component of the shift, which we discussed last week: Republican moderates are vanishing.

Interestingly, VoteView also provides a breakdown of polarization scores considering the geography of Democratic voters. We can overlay that data for the House, like so.

Northern House Democrats didn't change much in their politics, but Southern House Democrats changed significantly.

This chart looks a lot like one we made a few months ago, in which we tracked how far from the national vote margin states were in their presidential voting. Overall, the chart looked like this:

But we discovered that isolating the Deep South (Alabama, Georgia, Louisiana, Mississippi and South Carolina) eliminated much of the volatility on the Democratic side of the graph. In other words, except for those Southern states, states that voted Democratic did so fairly consistently compared to the rest of the country. And when those states started voting Republican, the Republican deviation from the national margin increased.

VoteView offers NOMINATE scores on another dimension of voting that was once a potent force in American politics, but is now generally absorbed into its overall measurements: the issue of civil rights for African Americans.

That score moved like this.

You will not be surprised to learn that Southern House Democrats were much more conservative on this issue, consistently.

Part of what happened is that southern Democrats inside and outside of Congress left the party in response to the party's shift on civil rights. If we look at the polarization trend in the Senate, where the strength of the southern Democratic caucus blocked civil rights legislation for years, the trend is remarkable. We've added a line below to show how the shift among Senate Democrats before 1950 matches the trend among Republicans in the last 40 years.

Our current polarization has complex causes that go deeper than our medium-term history. But that history correlates with what we're seeing today.