Generally when you think about the concept of a "wave election" you think about one Party winning a whole bunch of seats, possibly flipping both chambers, and basically leaving the other Party openly licking their wounds.
In this vein, there has been some liberal criticism of the President for not acknowledging his awful defeat in the midterms. People point to George W Bush's acknowledgment that Republicans took a "thumping" in 2006, and Barack Obama's acknowledgement that his Party took a "shellacking". Certainly many on the left would like to hear Trump say something similar.
But does what Trump does, and in this case Trump is claiming victory. In his mind, the increase in Senate seats (thus an easier path for him to nominate cabinet members and Judges) was a win "for him". He also seems pretty happy with himself for his the success of Senate candidates that he stuck his neck out for. The reality is that Trump may not actually see this like the left does. This obviously has rankled liberals to the point of another fit of rage.
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But let's be clear. In 2006, the Democrats took over both the House and the Senate and claimed a clear majority of the Governor's seats. More to the point, the thumping was so overwhelming that not a single Democratic incumbent lost a race in 2006. Like 2018, Republicans lost over 30 seats in the House, but they also lost 6 Senate and Governor's seats.
In 2010, the Democrats lost an amazing 63 seats in the House, 6 seats in the Senate, and 6 seats in the Governor's races. They also lost 680 seats in state legislative races. This was one of the most lopsided midterms in history. According to a metric created by Sean Trende, only the 1994 contract with America Gingrich revolution was a larger wave.
Drilling into the metric set up by Trende, a wave would come in at around 40 points (which is an objective complicated formula to measure gains by one party or the other). By Trende's definition, there have been only five "wave" elections. This would sort of make sense, as you have to make some distinction from a "wave" election to something that was just a "good result". Certainly everytime a chamber flips, you cannot demand that it was a "wave".
In fact, according to Trende's definition, the 2006 thumping fell short of being a "wave" calculating out to 30 points and placing 8th in terms of election results for one Party or the other. While some may quibble whether or not that election should be defined as a wave, Trende is being objectively consistent. By that measure, certainly if 2006 is not a wave, then 2018 isn't one.
By this same metric, the 2018 election scored a 24. It didn't even penetrate the top ten for election success. Much of this falls to the simple point that the Democrats lost four incumbents in the Senate and lost Senate seats overall, allowing the Republicans to increase their majority in that chamber. Moving down the ballot, while there were losses at the state legislative level, they were approximately half of what we saw in 2010. Certainly 2018 remains a good outcome for Democrats, but falls far short of previous elections, and likely short of many expectations going in.
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So how did Nate Silver determine that this was a "wave election"? Basically by ignoring the underlying concept that wave elections are about net results. Like everything that is Nate Silver, he decided to redefine traditional parameters and traditional methodology. For example, attempting to declare that voter turnout can be seen as one parameter, and then total votes as another parameter (as if they are not actually the same parameter). He cites things as voter turnout among hispanics, voter turnout of young people, as well as the fact that Independent broke for Democrats as being responsible for making 2018 a "wave" election. At the end of the day, Silver's opinions sounds more to me like a normal analysis of why Democrats had a "good" year, and fall way short of explaining why a Party that lost seats in one chamber, fell well short of expectations, but still had a historical "wave" result.
This is nothing new to the twenty first century political gamesmanship. If you cannot claim something is what you want it to be, you simply do away with the old definition and replace it with one you made up on the fly.