Counties providing each party with their biggest raw-vote margins in the various states (master list of posts)

Republicans, Alabama-Colorado
Republicans, Connecticut-Hawaii
Republicans, Idaho-Kansas
Republicans, Kentucky-Massachusetts
Republicans, Michigan-Montana
Republicans, Nebraska-New Mexico
Republicans, New York-Oklahoma
Republicans, Oregon-South Dakota
Republicans, Tennessee-Virginia
Republicans, Washington-Wyoming

Democrats, Alabama-Colorado
Democrats, Connecticut-Hawaii
Democrats, Idaho-Kansas
Democrats, Kentucky-Massachusetts
Democrats, Michigan-Montana
Democrats, Nebraska-New Mexico
Democrats, New York-Oklahoma
Democrats, Oregon-South Dakota
Democrats, Tennessee-Virginia
Democrats, Washington-Wyoming

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The movement of the title of the biggest county vote trove in given states for the two parties (as well as the relative magnitudes of the raw vote margins received out of those counties) can correlate interestingly with certain realignments. For example, the title of biggest Republican vote trove in Virginia moved to Arlington County in 1928, and then again in 1948, having in both cases previously generally been in small unionist Appalachian counties. In 1948, furthermore, Virginia was one of relatively few states where Dewey's biggest county raw-vote margin was bigger than Truman's. In 2004, the title of biggest Democratic vote trove in Virginia moved to Arlington County, having been in the city of Richmond in the prior six elections and having generally typically been in either Richmond or the Hampton Roads region for over a century. In all three cases, the shift either accompanied or portended a significant change in Virginia's voting habits. In 1928, Virginia voted Republican for the first time since 1872. In 1948, while Virginia did not vote Republican, it was the first single-digit winning margin for a Democrat in the state since 1896, and from the subsequent election (1952) on, Virginia would go on to vote Republican in 18 of 19 elections. Similarly, in 2004, Virginia did not vote Democratic, but in the subsequent election, Virginia began a run of now four straight elections of voting Democratic.

Similarly, in 2004, Kerry netted more vote out of Denver than Bush did out of El Paso County, Colorado, a reversal from 2000 and the only state in which this reversal occurred. From 2008 on, Colorado has, like Virginia, voted Democratic four times in a row (and counting). 

Conversely, even though Obama carried some counties in West Virginia in 2008 (the last election, as of now, in which any county in West Virginia has voted Democratic), his biggest vote trove was a county that had (to my knowledge) never been the biggest Democratic vote trove in West Virginia: Monongalia County, home of WVU.  

It is interesting to see how hard some counties that provided Democrats with their largest raw vote margins in their states fairly recently have shifted Republican. Logan County, WV, which provided Al Gore with his biggest raw vote margin in West Virginia, gave Trump 80.9% of its votes in 2020. McDowell County, which gave John Kerry his biggest raw vote margin in West Virginia, gave Trump 78.9%. Floyd County, Kentucky, which gave Al Gore his biggest raw vote margin in Kentucky (despite his carriage of the state's biggest county), voted 74.9% to re-elect Trump. Okmulgee County, which gave Al Gore his largest raw vote margin in Oklahoma in 2000 (the last time Democrats carried any county in Oklahoma), voted 67.6% to re-elect Trump.

In 1992, the biggest Republican county vote trove moved from a larger county to a smaller county in 20 states (it moved from a smaller to a larger county in one state, Florida). In 1996, the biggest Republican vote trove moved from a larger county to a smaller county in ten states (whilst moving from a smaller county to a larger county in five states). However, even in the five states where it moved from a smaller to a larger county, it did not return to the county where it had most typically been in the Cold War era: to Kent County rather than to Oakland County in Michigan, for example; to San Juan rather than to Bernalillo County in New Mexico; to Greene County rather than to St Louis County in Missouri.

Of course, 1992 and 1996 were fairly bad Republican defeats, but what was interesting was that even as George W. Bush won the 2000 election, the biggest Republican vote trove did not return to its traditional Cold War-era home in most of the states where it had moved to a smaller county in 1992 and/or 1996. It did not return to Jefferson County in Alabama, Fairfield County in Connecticut, Cobb County in Georgia, Marion County in Indiana, Jefferson Parish in Louisiana, Oakland County in Michigan, Olmsted County in Minnesota, St Louis County in Missouri, Bernalillo County in New Mexico, Suffolk County (or even Staten Island) in New York, Mecklenburg County in North Carolina, Washington County in Oregon, Montgomery County in Pennsylvania, Harris County in Texas, Salt Lake County in Utah, or Fairfax (or Henrico) County in Virginia. Bush moved the largest Republican vote trove to a larger county in only ten states; of the ten, he moved it to the county where it had habitually been during the Cold War in only Wyoming and Tennessee. (In New Hampshire, he moved it to Rockingham County, which is where it had typically been in the first half of the Cold War period, but which had been displaced by the larger Hillsborough County in the second half.) In Minnesota, Missouri, and Texas, he moved it to a county that was still smaller than the habitual biggest Republican vote troves of the Cold War era in those respective states. In Florida, he moved it back to Duval County, where it had been in 1992 (and before that in 1928), but by 2000 Orange County had outstripped Duval in terms of its voting weight.

In 2004, as Bush won re-election (by a somewhat larger margin than he had won in 2000), he moved the biggest Republican vote trove to a larger county in seven states, but to a smaller one in six states. In the states where he moved it to a larger county, it was, again, still not the habitual Cold War-era biggest Republican vote trove (although in Wyoming, it was the biggest county in the state). In Ohio and Nevada, Butler and Elko Counties, respectively, gave the Republican his biggest raw vote margin in their states for what appears to have been the first time. 

In 2008, the biggest Republican vote trove moved from a larger to a smaller county in 19 states (although in four of them--New York, Montana, Washington, and Wyoming--it simply moved back to the county where it had been in 2000). In 2012, Romney was able to move the Republican vote trove to a bigger county in only six states (and moved it to a smaller county in two); in those six states, he was able to move it back to its habitual Cold War-era home only in California (Orange), and was able to move it back even to its 2004 location only in California, Pennsylvania (Lancaster), Missouri (St Charles), Connecticut (Litchfield), and New Hampshire (Rockingham). 

In 2016, when Donald Trump won the election, he moved the biggest Republican vote trove to a larger county in nine states (but to a smaller county in six states). Only in New York did he return it to its habitual Cold War-era home (Suffolk). In Michigan (Macomb) and Florida (Lee), he moved it to a county it appears never to have been in before. 

In 2020, the biggest Republican vote trove moved from a larger to a smaller county in 11 states, and from a smaller to a larger county in only two, North Carolina and Nevada. Lyon County, Nevada became the GOP's biggest vote trove in its state for what appears to be the first time. 

Meanwhile, the biggest Democratic vote troves have tended toward being the largest county in every state (in line with the country's largest state, California, regularly being the biggest Democratic vote trove from 2008 on, and with the country's largest county, Los Angeles County, regularly being the biggest Democratic county vote trove from 2008 on). In 1976, the largest county in the state was also the largest Democratic vote trove in 18 states, and was the largest Republican vote trove in 17. In 2000, the largest county in the state was also the largest Democratic vote trove in the state in 20 states (and the largest Republican vote trove in only seven). In 2020, the largest county in 32 states was the largest Democratic vote trove, and the largest county in only one state (Montana) was the largest Republican vote trove in its state. The largest Democratic vote trove moved from one county to another in five states in 2020, and in every case, not only did it move to a larger county, but it moved to the largest county in the state.