Why were the 2018 US midterm elections not an overwhelming victory for the Dems?

When Donald Trump was elected president, I thought it would light a white-hot fire under the bottom of every left-leaning American with a pulse. After observing President Trump and his administration for two years (2016–2018), I thought quite a few right-leaning persons would be moving to the left.

I expected the 2018 midterms to be a Democrat tsunami, but it seemed more like a storm surge: big, but not stupendous. Was it too much to expect a complete upending of both houses? Do you remember what outcome you expected? Were you disappointed? Why wasn’t there an overwhelming victory for the Democrats?