I'd guess Harris County, Texas.
Harris is a fairly large county.
Possible leaders:
New York, NY 3570 (Bush votes/sq mile)
Kings, NY 1360
Arlington, VA 1142
Queens, NY 1119
San Francisco, CA 1096
Hudson, NJ 932
Bronx, NY 862
Richmond, NY 835
Nassau, NY 798
Suffolk, MA 753
Philadelphia, PA 748
Orange, CA 686
Milwaukee, WI 676
Union, NJ 665
Pinellas, FL 660
Bergen, NJ 653
DuPage, IL 602
Delaware, PA 575
Cook, IL 565
Ramsey, MN 562
Essex, NJ 530
Fairfax, VA 512
Hamilton, OH 502
Harris, TX appears to be around 40th a bit behind Dallas (306 versus 366).
