This evidence points to a lot of interesting conclusions. First, it underscores the nature of our political polarization. In the last 18 or so years, Republican-leaning states have become more Republican; Democratic-leaning states more Democratic; and the swing states haven’t budged (which helps account for why they are swing states!). Members of Congress generally try to reflect the views of their constituents, so these numbers help explain why polarization has increased quite substantially.
Second, it indicates the continuation of a long-running trend, one that dates back at least to the election of 1928. That year, Democrat Al Smith broke into the previously Republican bastion in the Northeast to win Massachusetts and Rhode Island. Meanwhile, Republican Herbert Hoover made “Hoovercrats” out of voters in the peripheral Southern states. That was the first manifestation of a long-running process, one in which the two regions basically flipped their partisan allegiances: the Northeast became Democratic while the South became Republican. These numbers suggest that this has been an ongoing process in the last two decades.
Additionally, note the West and the Midwest's move in a slightly different direction. Colorado, Nevada, and New Mexico – which had been safely Republican on the presidential level through the 1970s and 1980s – have become toss-up states. Meanwhile, in the Midwest, Iowa, Minnesota and Wisconsin – all of which went for Michael Dukakis in 1988 – basically split their votes in 2000 and 2004.
There’s one other pattern I’d like to point out here, and to do that I’m going to introduce the following map. This tracks each state's change in the partisan balance between 1993 and 2010. Deep red states have seen the GOP gain by 10 points or more over the Democrats while deep blue states have witnessed 10 point or larger gains for the Democrats. Light red and light blue states have had 5-to-9 point gains for the respective party.
Again, we see here the gains each party has made in the West and the Midwest. But take a closer look at the Border states – Arkansas, Kentucky, Louisiana, Missouri, and West Virginia. All of these states went for Bill Clinton two times in a row. Yet, they’ve all seen strong movements toward the Republican Party in the ensuing years – and though Barack Obama won a 7-point nationwide victory, he lost each of these states (by wide margins everywhere except Missouri).
Both parties have their problems in this map, but I think this is an under-discussed dilemma for the Democrats. Combined, these states will account for 37 electoral votes in the 2012 election, and they have basically drifted from being swing votes to solidly Republican ones. Add in the net of 6 electoral votes that have moved from safely Democratic to safely Republican states thanks to the 2010 reapportionment, and you’ve seen a shift of 43 electoral votes toward the Republicans. This, in turn, has put more pressure on the Democrats to perform in states like Ohio and Virginia, which historically have not been reliable supporters of the party.