The partisan streaks for each county ever over 500k:
Democratic since 1924 (Coolidge over Davis):
Bronx, NY
Kings, NY (Brooklyn)
New York, NY (Manhattan)
Ramsey, MN (St. Paul)
St. Louis City, MO
Suffolk, MA (Boston)
Democratic since 1928 (Hoover over Smith):
Wayne, MI (Detroit)
Democratic since 1932 (Hoover over FDR):
Philadelphia, PA (Yes, Philadelphia voted against FDR in the 1932 landslide.)
Republican since 1936 (FDR over Landon):
Tulsa, OK
Republican since 1940 (FDR over Willkie):
Knox, TN (470k)
Republican since 1944 (FDR over Dewey):
Lee, FL (Fort Myers)
Democratic since 1956 (Eisenhower over Stevenson):
Alameda, CA (Oakland)
Baltimore City, MD
Bristol, MA (New Bedford/Fall River)
Dane, WI (Madison)
Middlesex, MA
Milwaukee, WI
San Francisco, CA
Republican since 1956 (Stevenson over Eisenhower):
Greenville, SC
Democratic since 1960 (Nixon over Kennedy):
Multnomah, OR (Portland)
Republican since 1964 (Johnson over Goldwater):
Collin, TX
Denton, TX
El Paso, CO (Colorado Springs)
Kern, CA (Bakersfield)
Lancaster, PA (before that, last went Dem in 1856)
Oklahoma, OK
Sedgwick, KS (Wichita)
Utah, UT (Provo)
Republican since 1968 (Wallace over Nixon):
Montgomery, TX
Democratic since 1972 (Nixon over Mondale):
Allegheny, PA (Pittsburgh)
Cook, IL (Chicago)
Cuyahoga, OH (Cleveland)
Erie, NY (Buffalo)
Essex, NJ (Newark)
Fulton, GA (Atlanta)
Hennepin, MN (Minneapolis)
Hidalgo, TX (McAllen)
Jackson, MO (Kansas City) (before that, last went Republican in 1928)
Lake, IN (Gary)
(New) Orleans, LA
Prince George's, MD
Providence, RI
Queens, NY
Republican since 1976 (Carter over Ford):
Brevard, FL (Titusville)
Polk, FL (Lakeland)
Democratic since 1980 (Reagan over Carter):
Denver, CO
Democratic since 1984 (Reagan over Mondale):
Contra Costa, CA
DeKalb, GA
El Paso, TX
Essex, MA (Lynn)
Genesee, MI (Flint)
Hartford, CT
Honolulu, HI
Hudson, NJ
King, WA (Seattle)
Los Angeles, CA
Montgomery, MD
Norfolk, MA
Pierce, WA (Tacoma)
San Mateo, CA
Santa Clara, CA (San Jose)
Summit, OH (Akron)
Democratic since 1988 (Bush over Dukakis):
Baltimore County, MD
Bernalillo, NM (Albuquerque)
Broward, FL (Fort Lauderdale)
Bucks, PA
Camden, NJ
Clark, NV (Las Vegas)
Davidson, TN (Nashville)
Delaware, PA
Jefferson, KY (Louisville)
Miami-Dade, FL
Middlesex, NJ (New Brunswick)
Monroe, NY (Rochester)
Montgomery, PA
Nassau, NY
New Castle, DE (Wilmington)
New Haven, CT
Palm Beach, FL
Pima, AR (Tucson)
Plymouth, MA
Sacramento, CA
St. Louis County, MO
Shelby, TN (Memphis)
Snohomish, WA (Everett)
Union, NJ (Elizabeth)
Washington, OR
Westchester, NY
Worcester, MA
Democratic since 1992 (Bush over Clinton):
Bergen, NJ
Fairfield, CT
Franklin, OH (Columbus)
Passaic, NJ
Oakland, MI
Democratic since 1996 (Dole over Clinton):
Orange, FL (Orlando)
Republican since 1996 (Clinton over Dole):
Ocean, NJ
Spokane, WA
Democratic since 2000 (Bush over Gore):
Fairfax County, VA
Guilford, NC (Greensboro)
Marion, IN (Indianapolis)
Mecklenburg, NC (Charlotte)
Travis, TX (Austin)
Republican since 2000 (Gore over Bush):
Monmouth, NJ
Democratic since 2004 (Bush over Kerry):
Arapahoe, CO
Bexar, TX (San Antonio)
Dallas, TX
DuPage, IL
Fresno, CA
Hamilton, OH (Cincinnati)
Harris, TX (Houston)
Hillsborough, FL (Tampa)
Jefferson, AL (Birmingham)
Jefferson, CO
Kane, IL (Before that, last went Dem in 1856)
Lake, IL
Riverside, CA
San Bernardino, CA
San Diego, CA
San Joaquin, CA (Stockton)
Stanislaus, CA (Modesto)
Ventura, CA
Wake, NC (Raleigh)
Will, IL
Republican since 2012 (Obama over Romney), i.e. Obama-Trump counties:
Macomb, MI (before that, 2004 and then 1992)
Suffolk, NY (before that, 1992)
Democratic since 2012 (Romney over Obama), i.e. Romney-Clinton counties:
Anne Arundel, MD (Annapolis) (before that, 1964)
Chester, PA (before that, 2008 and then 1964)
Cobb, GA (before that, 1976)
Douglas, NE (Omaha) (before that, 2008 and then 1964)
Fort Bend, TX (before that, 1964)
Gwinnett, GA (before that, 1976)
Orange, CA (before that, 1936)
Salt Lake, UT (before that, 2008 and then 1964)
Democratic in 2020, last in 1916:
Johnson, KS (KC suburbs)
Democratic in 2020, last in 1948:
Maricopa, AZ (Phoenix)
Democratic in 2020, last in 1964:
Morris, NJ (Parsippany) (before 1964, last blue in 1912)
Tarrant, TX (Fort Worth)
Democratic in 2020, last in 1976:
Duval, FL (Jacksonville)
Williamson, TX
Democratic in 2020, last in 2008:
Kent, MI (Grand Rapids)
Democratic in 2020, last in 2012:
Montgomery, OH (Dayton)
Pinellas, FL (St. Petersburg)