This week, Democratic and Republican campaign operations acknowledged that the House playing field is expanding. The NRCC added another 10 House districts to its already robust list of 72 Democratic-held targets. And on the Democratic side, the House Majority PAC announced it would be reserving nearly $102 million in advertising in a whopping 51 media markets for the fall campaign. 

These moves suggest that both sides see the possibility of a Red Tsunami in 2022. 

But, given that districts are more polarized than ever, and that redistricting is likely to reduce the already small number of competitive districts by at least one-third, how big of a wave is possible?

First, a bit of history, in every midterm election since 2006, the party in the White House has seen its share of the two-party House vote (the popular vote for the House) drop by anywhere from 6.5 to 17 points from the previous presidential election. 

For example, in 2004, Republicans won the national House vote by 2.7 percent — 50.1 percent to 47.4 percent. But, two years later, Democrats won the national House vote 52.8 to 44.9 percent, an almost 11-point shift in Democrats' favor from the 2004 election. That 11-point shift resulted in a 31-seat pick-up in the House. In 2010, Republicans won the House vote by almost seven points, a 17 point improvement from their 2008 showing. They ended up winning a whopping 64 House seats that year. 

Currently, RealClear Politics shows Republicans ahead by 3.6 points in the generic ballot tracker. If that holds up through Election Day, it will represent a 6.6 percent positive shift to Republicans from 2020 (Democrats won the national House vote by 3.1 points in 2020).

So, what would a 6.6 point shift to the right look like? At a very crude level, we could say that it would shift the 2020 vote margin in every CD, about 7 points more Republican. So, for example, a district that Biden carried 52 percent to 45 percent (+7) would become a jump ball (50-50) in 2022. Or, a better way to think of it is that any district that Biden carried by less than 7 points would be in danger of flipping to the GOP.

The good news for Democrats is that (at this point) there are only 21 districts where Biden's margin was fewer than seven points. Even if we expand that universe to include districts Biden carried by 8-10 points, that universe of potentially vulnerable Democratic-held seats expands only slightly. 

However, the good news for Republicans is that they currently hold eight of those 21 seats that Biden carried by less than seven points. In other words, it would make some of the most vulnerable GOP-held districts's, like Rep. Don Bacon's Omaha-based NE-02 (Biden +6) and Rep. Dave Schweikert's Phoenix-based AZ-01(Biden +1.4), tougher for Democrats to pick off. But, these are "holds," not flips, which lowers the ceiling for GOP gains. 

Another factor that may cap GOP gains is that Democrats don't have to defend many Trump-won districts. Right now, Democrats represent only six districts (AZ-02, IA-03, WI-03, PA-08, ME-02, and OH-09) that Biden did not win. In 2018, Republicans were defending four times as many districts that Hillary Clinton had carried in the 2016 election. 

Finally, while Democrats picked up 40 seats in the 2018 election, they did not flip any CD in which Trump took 54 percent of the vote or more. In other words, the Blue Wave only breached the already most vulnerable portions of the GOP seawall. There aren't all that many districts held by Democrats that Biden carried with less than 54 percent; just 24 so far. This week, many of the newly added targets to the NRCC list are in districts that Biden carried with 56 to 59 percent of the vote. 

Of course, this kind of crude, back-of-the-envelope math doesn't always translate to actual wins and losses. Candidates and campaigns matter too. A wave election in 2018 wasn't enough to oust GOP Rep. John Katko from his blue-leaning upstate New York CD. And in Wisconsin, Democratic Rep. Ron Kind managed to hold onto his conservative CD in good Democratic years and bad ones. But, those 'unicorns' have become even rarer in recent years as fewer and fewer House incumbents are able to separate themselves from the party brand. 

Every metric we use to analyze the political environment — the president's approval rating, the mood of the electorate, the enthusiasm gap — all point to huge gains for the GOP this fall. But, those metrics are bumping up against an increasingly 'sorted' House with few marginal seats and few incumbents sitting in the "wrong district." As such, the more likely scenario for this fall is a GOP gain in the 15-25 seat range. 

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