There are loads of articles out there about how poorly the incumbent Admin's party does historically during the mid-terms. Here's one graphic that shows that since 1962, the incumbent party lost seats in the House all but twice and in the Senate 11 out of the 14 times.
So yeah, you don't have to call it a "backlash" but the Rs were expected to lose seats in the House and the Senate in 2018. They did much better than average. The number of seats in the House that the Dems picked up was a little on the high side, but by no means near a record, but the Rs gained enough Senate seats to tie the record for best performance since 1962 of the incumbent party
https://blogs-images.forbes.com/niallmccarthy/files/2018/10/20181009_Midterm_Performance.jpg
I think this actually goes to bolster my point.
In 2018 the GOP already had a large advantage via gerrymandering, and had great economic tailwinds and recent corporate tax break, and one of the most favorable Senate maps in recent memory (i believe they were defending just 11 seats, most in solidly red states), plus a president that was willing to go beyond the pale to push his party above the opposition. Yet they drumming they took in house races was far worse than is typical. Every advantage, yet they did worse.
Then look at 2020; incumbents typically have an advantage in re-election, and moments of crisis generally boost their approval ratings (the "rally around the flag"). Indeed, we saw that in most democratic countries. But they lost the WH, and the Senate.
One can debate how many seats an incumbent party "should" lose the next election cycle, but my broader point is that the GOP has already implemented most of these measures and they still haven't resulted in the kind of lock on power that others have suggested.
I still don't think 2018 was an unusually favorable year for them. For instance, 2012 would have been the best in the House because a fresh gerrymander is the most effective. By 2018, your 2010-census based gerrymander is "wearing off" as lots of people have moved and the voter composition in a district has changed a lot. So 2018 was actually one of the least precisely gerrymandered years of the decade. Only 2020 was, of course, more stale. 2022 will be more precisely and thoroughly gerrymandered, both because it is based off of fresher data and because of course, the Republicans have better tactics that are more widespread now than in 2010 when they implemented REDMAP. And of course, the state legislatures are implementing new and different measures to suppress votes further, so I think we will look back on 2018 and see that not only was it not quite the backlash year other midterm elections have been, but it was not even as favorable conditions as they are continuing to create for themselves. And why not? They are innovative and hard working and *scared* people, so they are very motivated and will not rest on their 2012 or 2018 or 2020 laurels. They still have a lot of gain they could realize though they will continue to need help from the Dems, independents and others that are opposed to continue shooting themselves in the foot, as they are wont to do.
*to clarify my point about 2020 laurels, since I presume most people think they lost shockingly horribly and it wasn't a good year for them, I will repeat that not onld did they not lose net seats in state legislatures, but actually one state legislature, New Hampshire, flipped back from blue to red. Down ballot was a blood bath for Dems and all of us who work in this space have been devastated for months about that. And state legislatures is where the rules are made for elections both on the state (and local) and federal level (unless HR1 passes, which does wrest some significant control, including on gerrymandering, from states). So the 2020 Republican successes in state legislatures is what is setting them up to potentially have such great years (better years than 2018 by a long shot) in 2022 and 2024.