Via Andrew Sullivan, Meng Bomin at Daily Kos has some cool maps with county-by-county Democratic primary and caucus results. One thing that this map makes clear is that pretty much the single most important factor in this race has been Hillary Clinton's astonishing weakness in the northwestern quadrant of the country. If you draw a line from Monterey, California, eastward to (approximately) Evansville, Indiana, and then north to Canada, you have an enormous chunk of the country in which there's very little red. So little, in fact, that Obama's delegate margins in these states have put the race essentially out of reach: Obama's net gain of 24 delegates from Minnesota exceeds Clinton's net gain from New Jersey (11) and Pennsylvania (12), even though those two states have about three and a half times the total number of delegates as Minnesota does.
There's been a ton of commentary on why Clinton has dominated the Rust Belt and why Obama has been able to run up such staggering margins among African-Americans. But there's been far less written on the more interesting and significant question of why Clinton proved so anathema to Upper Midwest, Rocky Mountain, and Pacific Northwest Democrats. Was it the caucus format? Regional political culture? Lingering resentment from Bill Clinton's presidency? Some combination thereof? Whatever the main cause, it made the difference.