One county made the difference in ten different states between that state going blue versus red in 2004 (or vice-versa in 2 cases). In most cases, the difference weren't trivial--for example, in Pennsylvania, a state that went for Kerry in 2004 by a 51.3 to 48.7 ratio over Bush (not including votes for other candidates) would've gone 52.6 to 47.4 Bush over Kerry without Philadelphia County. That's a greater margin than a number of red states--Colorado, Florida, Ohio, Nevada, etc. Some states have an even greater margin for Bush over Kerry if just one county is omitted.
Of course, that's not an entirely fair comparison to make--after all, most states have a mixture of big cities, suburbs, small cities, towns, rural areas, etc. But--it emphasizes how important it is to allocate resources to get out the vote in the larger cities (especially if they vote Democratic--which most larger cities do).
This diary is mostly here for the surprise...that one county made the difference in 10 cases between whether its state went for Kerry or for Bush. In 8 cases, Kerry was the victor, but in 2 cases (because of an extraordinarily conservative county), Bush won the state by 1 county. Here's the states/counties (and all percentages only include Bush and Kerry voters to make things simple)...
Delaware: KERRY 53.8, Bush 46.4
Delaware without New Castle County (Wilmington, etc.): Kerry 40.7, BUSH 59.3
Delaware's the oddball case here since New Castle County has the majority of the population (only 3 counties, 1 populous blue county, 2 generally rural red counties).
Illinois: KERRY 55.2, Bush 44.8
Illinois without Cook County (Chicago): Kerry 45.4, BUSH 54.6
Cook County alone turns a moderately red state into a moderately blue state.
Pennsylvania: KERRY 51.3, Bush 48.7
Pennsylvania without Philadelphia County: Kerry 47.4, BUSH 52.6
Oregon: KERRY 52.1, Bush 47.9
Oregon without Multinomah County (Portland): Kerry 47.1, BUSH 52.9
Portland and Eugene are well-known for being very progressive, while some parts of the state votes more like the Interior West. I'm sure the Jeff Merkley campaign is doing what it can to get out the Portland vote in the tight Senatorial race.
Wisconsin: KERRY 50.2, Bush 49.8
Wisconsin without Milwaukee County: Kerry 47.8, BUSH 52.1
Had Milwaukee County's voter turnout been just 11% less, Wisconsin would've gone red in 2004.
Michigan: KERRY 51.7, Bush 48.3
Michigan without Wayne County (Detroit): Kerry 47.8, BUSH 52.2
Nothing different here.
Washington: KERRY 53.6, Bush 46.4
Washington without King County (Seattle): Kerry 48.1, BUSH 51.9
Washington would be a swing state without Seattle; fortunately, Seattle keeps it fairly easily on the blue side.
Minnesota: KERRY 51.8, Bush 48.2
Minnesota without Hennepin County (Minneapolis): Kerry 49.3, BUSH 50.7
Not as big of a swing as in other states--but still there.
REVERSE CASES (Red states that would've been blue except for one county):
New Mexico: BUSH 50.4, Kerry 49.6
New Mexico without San Juan County (Farmington, etc.): Bush 49.4, KERRY 50.6
San Juan County is in the NW corner of the state and has both mid-sized towns like Farmington and rural territory. It's solidly Republican.
Iowa: BUSH 50.3, Kerry 49.7
Iowa without Sioux County (Orange City but NOT Sioux City): Bush 49.9, KERRY 50.1
So...there's the list. Ten states basically won (one way or the other) by one county. Pretty amazing.
Of course, I must play my own devil's advocate here--one can't just focus on the big cities (after all, if you ignore the smaller towns and don't register enough Democrats there, the other side will win a lot of seats in the state houses, local elections, etc.--but--the big cities are going to carry a lot of votes, and in most of the cases above, one big city was indeed the difference between red and blue). Of course, voter suppression must be stamped out anywhere--but its impact is felt greatest if it occurs in a big city.