Midterms are referendums on the sitting President. Right now Trump has a 38-39% approval rating and just failed to get his healthcare bill (which hovered between 12-19% approval) through congress. Right now the Democrats are leading by about 8 points according to the 538 tracker for 2018.
The GOP could save their majorities if they actually moved onto other popular agenda items like tax reform and infrastructure.
Conveniently for Democrats, Trump's current approval ratings is in the same exact range Bush's was during the 2006 midterms.
Just because GWB had 2 unpopular wars raging on, a botched disaster relief operation and oodles of scandals for his party does not mean it would take an equivalent of that for Democrats to have another large wave under a GOP president, not to mention that people's interpretations of what is "equivalent" would vary quite a bit. Given how things have changed since 2006, it's possible that state-level gains are more limited and obviously Democrats are much more likely to end up with a slim House majority if they can get one at all, but there are already a ton of signs that a wave is building up. I don't get how some people can't see this. This is supposed to be the best part of Trump's presidency - the beginning, and it usually goes downhill from here. Given that Trump is, well, Trump, I doubt he will buck that trend.
