Major counties that could have been 2024 Democratic beachheads

These are major counties that voted for Trump twice, that voted for Romney in 2012, and whose last two losing votes prior to 2020 were for Republicans, but that Trump carried more weakly in 2020 than in 2016, and which Trump carried by a smaller margin than Bush carried Salt Lake County, Utah by in 2004 (the largest Bush margin in a major county that Obama carried in 2008). (Bush carried Salt Lake County by 22.03% in 2004.) Had Trump won reelection, these are counties that could potentially have been 'beachhead' counties for the Democratic nominee in 2024. (Their existence also shows that Trump did retain some suburban support in 2020, as Bush did in 2004.)

Madison, AL (194,767) (Huntsville)
2016: 54.8%-38.5%
2020: 52.8%-44.8%

Mobile, AL (183,164) (Mobile)
2016: 55.1%-41.8%
2020: 55.2%-43.3%

Pinal, AZ (248,874)
2016: 56.2%-37.0%
2020: 57.7%-40.5%
(although Trump's 57.7% was the highest Republican vote share in Pinal since 1972)

Kern, CA (305,226) (Bakersfield)
2016: 53.1%-40.4%
2020: 53.9%-43.7%

Placer, CA (235,017)
2016: 52.0%-40.2%
2020: 52.1%-45.5%

Tulare, CA (146,832)
2016: 51.0%-41.7%
2020: 52.8%-45.0%

El Dorado, CA (116,138)
2016: 51.8%-38.3%
2020: 53.2%-44.4%

El Paso, CO (378,849) (Colorado Springs)
2016: 56.2%-33.9%
2020: 53.5%-42.8%

Douglas, CO (231,599)
2016: 54.7%-36.6%
2020: 52.4%-45.2%

Weld, CO (166,968)
2016: 56.6%-34.4%
2020: 57.6%-39.6%

Sussex, DE (129,352)
2016: 59.0%-37.1%
2020: 55.1%-43.8%

Lee, FL (393,899) (Fort Myers)
2016: 58.1%-37.9%
2020: 59.0%-39.9%

Brevard, FL (360,764)
2016: 57.1%-37.6%
2020: 57.4%-41.0%

Pasco, FL (301,970)
2016: 58.4%-37.0%
2020: 59.3%-39.3%
(although Trump's vote share was the highest of any Republican's since 1984)

Sarasota, FL (270,575) (Sarasota)
2016: 53.8%-42.3%
2020: 54.7%-44.3%
(although Trump's vote share was the highest of any Republican's since 1988)

Manatee, FL (216,958)
2016: 56.4%-39.4%
2020: 57.5%-41.5%
(although Trump's vote share was the highest of any Republican's since 1988)

Lake, FL (211,314)
2016: 59.5%-36.6%
2020: 59.6%-39.5%

Escambia, FL (170,372) (Pensacola)
2016: 57.6%-37.3%
2020: 56.6%-41.5%

Ada, ID (259,700) (Boise)
2016: 47.9%-38.7%
2020: 50.3%-46.4%

McHenry, IL (163,874)
2016: 49.8%-42.2%
2020: 49.9%-47.5%

Sangamon, IL (104,751) (Springfield)
2016: 50.8%-41.6%
2020: 51.1%-46.7%

Hamilton, IN (193,999)
2016: 56.0%-36.7%
2020: 52.4%-45.6%

Allen, IN (169,082) (Fort Wayne)
2016: 56.5%-37.3%
2020: 54.5%-43.3%

Sedgwick, KS (224,862) (Wichita)
2016: 54.4%-36.3%
2020: 54.7%-42.9%

Jefferson, LA (193,733) (Metairie)
2016: 55.3%-40.6%
2020: 54.6%-43.6%

Harford, MD (148,186)
2016: 58.3%-35.2%
2020: 54.6%-42.6%

Ottawa, MI (168,713)
2016: 61.5%-31.3%
2020: 59.8%-38.4%

Anoka, MN (211,132)
2016: 50.3%-40.6%
2020: 49.7%-47.8%

St Charles, MO (226,115)
2016: 59.9%-33.8%
2020: 57.8%-40.3%

Greene, MO (144,902) (Springfield)
2016: 59.8%-32.7%
2020: 59.0%-38.8%

Clay, MO (129,133)
2016: 51.7%-40.8%
2020: 51.0%-46.9%

Monmouth, NJ (378,390)
2016: 52.5%-43.2%
2020: 50.7%-47.9%

Oneida, NY (101,858) (Utica)
2016: 56.5%-37.1%
2020: 56.8%-41.3%
(although Trump's vote share was the highest of any Republican's since 1984)

Cabarrus, NC (117,227)
2016: 57.7%-38.1%
2020: 53.9%-44.5%

Lake, OH (130,782)
2016: 54.8%-39.6%
2020: 56.0%-42.5%

Delaware, OH (126,374)
2016: 54.5%-38.7%
2020: 52.5%-45.7%

Oklahoma, OK (294,740) (Oklahoma City)
2016: 51.7%-41.2%
2020: 49.2%-48.1%

Tulsa, OK (266,678) (Tulsa)
2016: 58.4%-35.6%
2020: 56.5%-40.9%

Cleveland, OK (119,778)
2016: 57.1%-35.5%
2020: 55.7%-41.6%

Douglas, OR (127,165)
2016: 49.3%-40.7%
2020: 50.2%-46.8%

Lancaster, PA (280,239)
2016: 56.3%-37.2%
2020: 56.9%-41.2%

Berks, PA (205,540) (Reading)
2016: 52.5%-42.6%
2020: 53.2%-45.0%
(although Trump's vote share was the highest of any Republican's since 1988)

Cumberland, PA (141,595) (Carlisle)
2016: 55.9%-38.1%
2020: 54.3%-43.8%

Greenville, SC (258,155) (Greenville)
2016: 59.4%-34.6%
2020: 58.1%-39.9%

York, SC (144,050)
2016: 58.4%-36.4%
2020: 57.4%-41.0%

Berkeley, SC (104,458)
2016: 56.1%-38.6%
2020: 55.0%-43.3%

Knox, TN (220,556) (Knoxville)
2016: 58.5%-34.8%
2020: 56.5%-41.5%

Hamilton, TN (171,113) (Chattanooga)
2016: 55.3%-38.8%
2020: 53.8%-44.1%

Rutherford, TN (143,878) (Murfreesboro)
2016: 60.1%-34.2%
2020: 56.6%-41.2%

Collin, TX (490,856) (Plano)
2016: 55.2%-38.6%
2020: 51.3%-46.9%

Denton, TX (417,964)
2016: 57.1%-37.1%
2020: 53.2%-45.2%

Brazoria, TX (154,984)
2016: 60.1%-35.7%
2020: 58.4%-40.2%

Bell, TX (127,375) (Killeen)
2016: 54.3%-39.5%
2020: 53.3%-44.8%

Spokane, WA (295,430) (Spokane)
2016: 48.1%-39.7%
2020: 50.3%-46.0%

Benton, WA (103,033)
2016: 56.5%-31.5%
2020: 58.6%-37.6%

Waukesha, WI (267,996)
2016: 60.0%-33.3%
2020: 59.6%-38.8%

Brown, WI (144,017) (Green Bay)
2016: 52.1%-41.4%
2020: 52.7%-45.5%

Outagamie, WI (108,022) (Appleton)
2016: 53.1%-40.5%
2020: 54.1%-44.1%

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The above are listed by state; the five largest overall are:

Collin, TX (490,856) (Plano)
Denton, TX (417,964)
Lee, FL (393,899) (Fort Myers)
El Paso, CO (378,849) (Colorado Springs)
Monmouth, NJ (378,390)

Collin and Denton have been on a trend away from the GOP beginning in 2004, and El Paso, beginning in 2008. Monmouth was a historically Republican county that voted for Clinton in '96 and Gore in '00; it hasn't voted Democratic since 2000, but aside from the 9/11-influenced election of 2004, Republican margins have remained relatively small (although Trump had improved on Romney in 2016). The GOP vote share in Lee County has risen for three elections in a row, and Trump's 59.0% is behind only Bush's 2004 59.9% as the highest GOP vote share this century. 

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This list is likely excessively expansive, and probably could have used some further limiting criteria (such as Trump's vote share not being the highest Republican vote share this century, or Biden's vote share having been above Obama's 2012 vote share). After all, Trump's national margin did contract by 2.36% in 2020, and this list includes almost every major county Trump carried in which his margin was less than 22.03%, and which voted for Romney and whose last two losing votes prior to 2020 were for Republicans (two of the few exceptions were Polk, FL and Nueces, TX). However, the chief criterion will be which ones become either laggard Democratic beachheads in 2024 (if the Democratic nominee wins that year, or even loses narrowly), or become Democratic beachheads the next time the Democratic Party takes the White House back from the GOP (or even comes close to doing so). Some counties that have particularly been of note in the last two elections (in the context of the erosion in Trump's suburban support) have been Douglas, CO, Ada, ID, Hamilton, IN, Cabarrus, NC, Delaware, OH, Cumberland, PA, Collin, TX, Denton, TX, and Waukesha, WI.

(It is also interesting that, even though Trump's national margin did contract in 2020, if we counted all the major counties that Trump carried in 2020 by a margin of less than 22.03%, there would be a number in which his margin expanded--for example, Stark, OH, whose last two losing votes had been in 2004 [Kerry] and 1976 [Ford]; Trumbull, OH, whose last two losing votes were in 2004 [Kerry] and 2000 [Gore]; Volusia, FL, whose last two losing votes had been in 2012 [Romney] and 2004 [Kerry]; and of course the two major counties Trump flipped in 2020, Mahoning and Lorain, OH, both of whose last two losing votes were in 2004 [Kerry] and 2000 [Gore]. In other words, across the board Trump seems to have done better at improving his position in 2020 in places that used to be more Democratic or battleground regions.)