I'm hardly one to disagree with Fivethirtyeight.com's Sean Quinn about the rising significance of the Mountain West. (I prefer "Interior West," because some parts of this subregion aren't very mountainous, and some of the coastal states have mountains but aren't in this category of eight non-Pacific states.) There's a lot of action in the Interior West, and Democrats are making huge gains of late. So if Sean is merely saying the Interior West is emerging as a new swing region, I'd of course agree.
But the swing region in America remains the Midwest. I don't think there is much of a dispute about this, or at least there shouldn't be. In 2004, it was home to five of the 12 states decided by 5 points or less. It was the region where the electoral vote was most closely divided in 2008, and is the most divided region in Congress following the 2008 results. Even if you look at the nine states Barack Obama flipped between 2004 and 2008, the three Southern wins were very narrow or razor-thin, and the three Southwestern wins were very comfortable. But it was the three Midwestern wins that were themselves a mixed bag (Iowa comfortably, Ohio narrowly, Indiana razor-thin) of competitiveness.
More specifically, the bellwethers for presidential politics are what I would label "the Great Eight" Midwestern states that border either a Great Lake or the Mississippi River: Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio and Wisconsin. In every election from 1920 to 2008 -- 23 in all -- the candidate who carried four or more of these eight states won 22 times. And the exception? George W. Bush in 2000, when he finished second in the popular vote. That means that the candidate who carried at least four of the Great Eight has won the popular vote in 23 consecutive presidential contests. You don't find many patterns as reliable as that.
The Interior West is emergent, and growing in size. But, while the Midwest is losing congressional seats and thus electors, it remains for now the critical region. And it is no surprise that the winner in 2008 was a candidate from the largest city in the Midwest -- or that he just so happened to defeat a candidate from the largest city in the Interior West.