Dueling landslides by Clinton and Obama
The New York Times Book Review has a piece on the "clustering" of people with similar political views. The Big Sort, by Bill Bishop and Robert G. Cushing, explains (in possibly alarmist terms) how geographic mobility in the United States is leading to almost monolithic communities of liberal Democrats (or conservative Republicans). One statistic mentioned in Scott Stossel’s review is that only 26 percent of the population lived in "landslide counties" during the 1976 presidential election but nearly 50 percent of us fell into that category in 2004. (Stossel doesn’t define "landslide.")
The 2008 Democratic primaries certainly support this thesis, though racial divisions explain much of the divide between Clinton counties and Obama counties. Looking at primaries and caucuses where both candidates were on the ballot (i.e., excluding Michigan) and county-by-county results are available, we see that Obama has won 827 counties by more than 10 points and Clinton has won 1,229 counties by more than 10 points. The candidates were separated by less than 10 points in only 539 counties, or only about one-fifth of the counties that have voted so far in what is, overall, an extremely tight popular-vote race.
In the following states, there was a swing of more than 100 percentage points between the strongest Clinton county and the strongest Obama county. Differences may be more exaggerated in caucus states, since turnout depends more on get-out-the-vote efforts by the campaigns. (Except where noted, numbers are percentages, not raw votes.)
COLORADO caucus: Clinton wins Kit Carson County (Kansas border) 64-27; Obama wins San Miguel County (Utah border) 86-13.
GEORGIA primary: Clinton wins Murray County (Tennessee border) 81-14; Obama wins Clayton County (outside Atlanta) 82-17.
IDAHO caucus: Clinton wins Lewis County (panhandle) by 10 caucus votes to 7; Obama wins Clark County (east) with all 6 caucus votes.
LOUISIANA primary: Clinton wins Cameron Parish (Texas border) 69-18; Obama wins Orleans Parish (New Orleans) 75-23.
MISSISSIPPI primary: Clinton wins Tishomingo County (northeast) 82-13; Obama wins Jefferson County (Mississippi River) 88-11.
MISSOURI primary: Clinton wins Dunklin County ("the Bootheel") 78-18; Obama wins the city of St. Louis 71-27.
NORTH CAROLINA primary: Clinton wins Graham County (Tennessee border) 77-18; Obama wins Durham County 75-23.
VIRGINIA primary: Clinton wins Buchanan County (Kentucky border) 90-9; Obama wins the city of Petersburg (near Richmond) 84-15.