NOTE:
Alaska is omitted because there is no way to accurately determine how each borough voted.
For New York, I did New York City (all five boroughs) instead
Margins:
20% - <5%
30% - 5%-10%
40% - 10%-15%
50% - 15%-20%
60% - 20%-25%
70% - 25%-30%
80% - 30%-40%
90% - 40%+
The closest biggest counties that went for Obama were Harris County, TX (D+0.08%), Hillsborough County, NH (D+1.12%), East Baton Rouge Parish, LA (D+5.23%), and Jefferson County, Alabama (D+5.97%). The closest biggest counties that went for Romney were Cass County, ND (R+2.90%), Douglas County, NE (R+3.02%), and Minnehaha County, SD (R+7.40%).
The strongest biggest counties for Obama were Philadelphia County, PA (D+71.27%), NYC, NY (D+63.41%), and Multnomah County, OR (D+54.72%). The strongest for Romney were Greenville County, SC (R+27.75%), Laramie County, Wyoming (R+24.32%), Yellowstone County, MT (R+20.49%), El Paso County, CO (R+20.38%), and Salt Lake County, Utah (R+20.22%).
I will add the map with the remained of the states later.