Hmm, any Blue Wave (if it exists) should dissipate by 2020 in the same manner 2012 didn't follow along on what happened in 2010. I can see the Dems winning the Governor seat, but losing the State House and missing out on the Senate in 2020.
Why would that happen?
Seriously. You know elections don't just replay the same way forever for no reason, right? I mean, Trump could win again in 2020, but you ought to make an argument for it other than "but 2012," which is a reallllllllly lazy way of thinking about this.
If you want to go the 2012 route though, think about it this way: Obama lost a little less than half his first election's winning margin. What happens if the 2020 Democrat wins by almost double Clinton's margin, replicating 2012-like parameters? Trump loses, and it won't be that close either. You can't just take "2020 = 2012" and only take the parts that fit what you want to happen. That's not how any of this works.
What's wrong with taking historical parallels? '94 was a Dem bloodbath, but Hillary's husband hung on just 2 years later. Same with '82 and '84 reversed. Sure, DJT may be a one-termer, but it's common for the outparty to let out their frustrations in the midterms and then tire out by the time the Presidential campaign rolls around. It took a freakin' hostage crisis and dizzying gas prices/inflation to sink Carter.
Because there's an important difference between previous midterm waves and this one. In 1982 and 2010, the president had room to improve. The wave against them wasn't 100% caused by the economy being terrible, but that was a major factor, and when it recovered, the president rose in popularity. That just doesn't apply to this election. Trump's not loathed because the economy is sh**tty. He's loathed because he's corrupt, stupid, and broadly incompetent, and he can't really fix those.
In other words, if the economy stays good, Democrats will likely do about as well in 2020 as they did in 2018. If it gets worse (which it almost certainly will), they will do even better.