Nate Silver over at 538 (via Andrew Sullivan):
That's right: just five states, collectively containing about 2 percent of the American population, have statistically significant pluralities of adults identifying themselves as Republicans. These are the “Mormon Belt” states of Utah, Idaho and Wyoming, plus Nebraska, plus Alaska. By contrast, 35 states are plurality Democratic, and 10 states are too close to call.
Now then for a couple of caveats. Firstly, Gallup's numbers consist of interviews with all adults — not registered voters, and certainly not likely voters. Depending on the particular application that we're using this data for, that may be helpful or unhelpful. What this perhaps indicates, however, is that even after all the millions of new voters that the Democrats registered and brought to the polls in 2008, there are still probably some marginal gains to be had, particularly in areas like the deep South that the Obama campaign did not really concentrate in.
Secondly, these totals include “leaners” — independents who lean toward one party or another, but don't identify themselves as such. This tends to increase the Democratic margin by a couple of points.
Thirdly and perhaps most importantly is a point that both Michael Barone and I have raised at various times: one consequence of the Democratic coalition being larger, particularly as it tends to include a miscellany of groups that don't always see eye-to-eye with one another (African-Americans, Hispanics, coastal liberals, union workers, young voters, etc.), is that it is more difficult to harness the entirety of that coalition in national elections.