When Hillary Clinton won Orange County CA in 2016, she was the first Democrat to do so since the Great Depression (1936).
Two years later, it’s looking like Orange County won’t have a single Republican in Congress for the first time since the 1930s.
If Democrats defeat Rohrabacher, Walters and Kim, it will be the first time since the 1930s that Orange County has no Republican in Congress.
Nate Silver’s latest projections have all those races as more likely than not to flip blue:
That’s how quickly a major county that was once a Republican stronghold (the “Orange Curtain”) flipped from red to completely blue.
Having lost The OC for the first time in a lifetime, the largest counties by GDP that that the Republicans won with Trump in 2016 were (in order) Maricopa County AZ (Phoenix), Tarrant County TX (Fort Worth), and Suffolk County NY (The Hamptons in eastern Long Island).
Image from WaPo (share of total GDP):
WaPo: Donald Trump lost most of the American economy in this election
You can see immediately what's going on: With the exceptions of the Phoenix and Fort Worth areas, and a big chunk of Long Island, Clinton won every large-sized economic county in the country.
Two years later, in Senate races in those states, the Democrats won all three counties:
-Kyrsten Sinema almost certainly won Maricopa County (currently 50.6-47.2)
-Beto O’Rourke won Tarrant County (49.9-49.3)
- Kirsten Gillibrand won Suffolk County (54.2-45.8)
If those three counties start to go blue, what are the biggest remaining red counties?
The only other one that's even large enough to be labeled on that WaPo map (although seven counties represented by thin unlabeled boxes appear to be larger) is Kent County MI (Grand Rapids), which Debbie Stabenow lost (49.6-48.2)… It should be noted that Michigan’s new governor Gretchen Whitmer did win Kent County.
But as you can see from the CNN visualization here, after those big three 2016-red/2018-blue counties, the rest are quite a bit smaller... the next largest may actually be Pinellas County (St. Petersburg) or Duval County (Jacksonville) in FL. Even those counties turned blue in this year’s Senate race, with Bill Nelson currently leading in both (Pinellas 52.6-47.4, Duval 50.8-49.2). So it could be that the five largest Trump counties all turned blue!
Image from CNN (GDP growth):
tl;dr
64 percent of America's GDP was from blue counties after the 2016 election, after the 2018 midterm election that percentage has to be even higher with the three (possibly five) largest red counties all going blue.
[ Relevant articles on 2016 election results and county GDP: WaPo, Brookings, CNN ]