The number of counties won by Democrat & Republican presidential candidates in key 2020 battle ground states since 2000 to 2016 presidential general elections gives insight into the effort needed by each party to win the elections.  Based on the data the Democrats almost always just need to win 1/4th to 1/5th the number of counties to carry the elections like in 2008 and 2012.  Out of the 3000+ counties, the President Obama only had to win 708 counties in total to win his re-election in 2012, which was 172 (880)  less than his precious years.  Hillary Clinton only won 497 counties in 2016.  Mccain and Mitt Romney both won over

The line shows clearly that President Obama in 2008 won more counties than any democrat including himself (in 2012).  Hawaii and Arizona have remained steady with the number of counties won at 4 which shows a horizonal line at the bottom which serves as a great contrast for gains and loses by a party.  

In the battleground states of Arizona, Michigan, Minnesota, North Carolina, Pennsylvania, Wisconsin the chart above shows in no state did they do well post 2008. Since 2000, in 2016 they held the county count in North Carolina, Arizona, but all the other states they are won less than 2000.   Biggest drops is in the state of Iowa, 51 counties won in 2008 to 6 in 2016 as compared to 28 in 2000.  It was on a downward trend as in 2012, President Obama got 28.  In Wisconsin, where they dropped from 59 in 2008, 32 in 2012 and 12 in 2016 which is less than their 2000 & 2004 of 26.  Michigan, 46 counties in 2008, to 19 in 2012 and 11 in 2016, dropping from 32 in 2000.  Minnesota also saw the same trend with 2008  won 41 which dropped to 28 in 2012 and dropped to its lowest to 9 counties, which is lower than the 17 counties in 2000.    North Carolina and Pennyslvania shows the same trend but at a lower range.

The exception of the 2020 battleground states is Illinois, where the Democrat party saw a big drop from 2008, where they won 46 counties, dropping to 11 counies, fairing even worse than the 23 in 2012 and  24 in 2000.

The above chart makes it easy to compare year over year by each state when you move cursor over a particular state name you will see the listing of number of counties won from 2000 to 2016 general election.   For example. Wisconsin shows from 2000,2004,2008,2012,2016 as 26,26,59,32,12, while a state like Ohio 16,16,21,16,7 for the same years.  In Wisconsin, it shows there is quite a lot of variation in the number of counties, while in state like Ohio, the variation is very small.  These patterns tend to be seen in multiple states.

In the above chart, adding the vote difference between the Democrat candidate votes versus the  Republican candidate, in only the counties won by the Democrat candidate for that state, one sees the win contribution by these Democrat leaning counties.  For example, the state of California, in 2000, the Democrat candidate "Al Gore" won 20 counties contributing 1.8M votes for the candidate in the state. In 2004, John Kerry, won the same number of counties,  contributing 2.1M votes. President Obama, won 34 counties, getting 3.5 M votes and in 2012 he won 31 counties with 3.3M votes.  In 2016, Mrs. Clinton, won less counties than President Obama in 2008, but got 1.2M more votes than him in the 33 states she won, of 4.5M votes from these counties in the state.  

In Florida, we see the almost a similar trend as in California, only the difference being, the number of counties lost in 2016 is almost half as in 2000, but the number of vote difference in these 9 counties is double that of that in 2000.  So, in 2000, Democratic party won 16 counties, with a vote difference of 500K votes.  In 2004, the party won 9 counties over the Republicans giving then again a 500K vote difference, however the same number of 9 counties in 2016 gave them a vote difference of a 1M.  In 2008 and 2012, President Obama got 820K votes difference in the 15 and 12 counties he won in 2008 and 2012 respectively. Hillary Clinto did better than President Obama, John Kerry and V.P Al Gore with respect to brining out the votes in smaller number of counties.

Contrast the California data, with a state like Ohio ,  In Ohio, ,the Democratic party, won 16 counties in 2000,2004 with vote difference being 315K , 468K. Presidet Obama, in 2008 200K more voters than John Kerry and, in 2012, for his re-election bid got 650+ votes.  In 2016, Mrs. Clinton got as much as John Kerry did.  This type of analysis gives one the insight into Democratic party base in a state.  

Looking at these states, it looks like majority of these states have been in a downward trends since 2008. As seen earlier, since 2008 the is a sizable shift downwards in key states, but this trend continued through 2012 all way to 2016, except in a California where they gained and kept the number of counties won in  Maryland, Massachusetts, Nevada, Rhode Island.