Going back to the year 2000, electoral advantages between the GOP and DEM was split roughly evenly, where either party could effectively guarantee around 220 electoral votes and then the remaining 80-100 were up for grabs. Going back to 2000, the only nominee to not get more than 220 was when McCain had the unfortunate situation of trying to represent the GOP when the economy collapsed near the end of the Bush years..... Because of this near-certainty, a handful of swing states held great importance in national elections where losing 3 or 4 of them effectively ensured defeat, due to how evenly split the electoral power both sides had on average.
That is shifting as we go into 2020 and beyond though.
The states of Texas, Arizona, and Georgia collectively have 65 electoral votes between them. That number is likely only going to climb going into the future as states in the south tend to have greater growth compared to states in the north. The problem though for the GOP is that these 3 states, which have gone Republican in every recent election without the GOP having to sink resources into those states to defend them, are now drifting into swing-state territory, assuming they're not swing states already.
Last 3 polls from Arizona shows Biden +7. Trump +1, Biden +4
https://www.realclearpolitics.com/epolls/2020/president/az/arizona_trump_vs_biden-6807.html
Last 4 polls from Texas shows Tie, Biden +1, Trump +6, Trump +1
Last 4 polls from Texas shows Tie, Biden +1, Trump +6, Trump +1
Sparse polls from Georgia show Trump +7 and Trump +8, but neither of those polls were taken in 3 months, and historically Georgia went Republican by an averega of 15% during the bush years whereas Romney and McCain only carried the state in high single digits
These three states are the three that have historically been the largest safe states for the GOP in presidential elections..... North Carolina usually breaks GOP but in the last 3 elections in a row have been decided by less than 3%, the Great Lakes states like Ohio, Penn, Mich, and Wisc tend to lean blue or be swing states, while no other safe GOP state in the Midwest carries more than 10 electoral votes.
If this trend holds where it currently is, meaning from now on everything stays the same and doesnt change further, this means that the largest 'safe' state the GOP can count on would be Tennessee at 11 electoral votes.
Democrats have SIX states larger or equal to that size that are safe Dem territory. CA at 55, NY at 29, IL at 20, NJ at 14, WA at 12, MA at 11..... Seven if you count Virginia at 13 which even Hillary won by 5% where the last two polls show Biden carrying by a massive 11 points.
If we put Colorado, New Mexico, and Virginia into Democrat territory, and throw in Maine in there as well which is only 4 delegates total so whatever, then going into the 2020 election Dems can reasonably rely on 213 electoral votes at minimum, needing only 57 more to hit the magic 270 number..... 213 at minimum is pretty on par for recent presidential elections going back to the year 2000. https://www.270towin.com/
The GOP on the other hand, with Texas, Georgia, and Arizona all drifting towards swing state territory if they arent there already, can only safely rely on 127 electoral votes, less then HALF what is needed to hit 270. Even if you think Georgia will stay with the GOP for at least the next 3 election cycles, Georgia's 16 only puts the GOP at 143, which is barely over halfway to the 270 number needed to win the presidency, and 70 electoral votes behind where Dems are already starting off at 213.
If the GOP win 6 the smaller swing states that have less than 11 electoral votes (Nevada, Iowa, New Hampshire, Minnesota, Wisconsin, Arizona) That still puts them 40 electoral votes behind the DEMs 213 to 173....... Add in Georgia and North Carolina on top of that and they STILL trail 213 to 204
For those keeping track, the GOP would have to win the 8 of the smallest swing states just to pull about even with Dems that are relying on just safe Dem states.
Of the 5 remaining large swing states: Florida, Texas, Michigan, Pennsylvania, and Ohio, lets give Ohio to the GOP and Michigan to the Dems.... Ohio went 8 points for Trump in 2016 (the largest margin of this bunch) while Michigan went Democratic 6 of the last 7 elections where the one time it did go GOP it went for Trump by 0.2% (3 most recent polls in Michigan also show Biden leading +8, +2, and +12 so you can argue this state of the 5 mentioned is the one most likely to go Dem)
With Ohio going GOP and Michigan going DEM, that puts the electoral count at 229 to 222 Dems leading. Whoever wins two of the remaining 3 swing states (Texas, Florida, Pennsylvania) would go on to clinch the election.
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To summarize all the info above, the GOP is getting to the point where it would need to win about 8 to 11 swing states just to barely win a presidential election, while the Dems only really need 3 to 5. This is due to the fact that once reliable GOP states of significant size (Arizona, Texas, Georgia) are trending more towards swing state territory or are already in swing state territory, while no other safe Dem state has drifted into swing state status at at the same time (Michigan, Pennsylvania, Wisconsin, nor Virginia were ever guaranteed to go Dem in previous elections,
This is IF things stay the same. If Texas and Arizona continues to get bluer due to population patterns, the way Virginia clearly has, then Dems would only need any 2 additional swing states to clinch an election whereas the GOP would need at least 10. That's just not sustainable going into the near or long term future, and something will need to change in order for the GOP to have a fighting chance in presidential elections going forward.