I'm going to take issue with this sentiment. I'll bet dollars to doughnuts, that the vast majority of the disapproval of the congress-critters is the frustration that one's party doesn't accomplish what they want accomplished. In other words, conservatives are frustrated with the republican elites for not pushing through the entire Trump agenda (see also Paul Ryan and Mitch McConnell) more quickly and approve more judges quicker. The liberals are frustrated that the democrats weren't able to stop certain portions of the Trump agenda (Tax cuts passed, regulations reduced, withdraw from the Paris Accord, etc.) and aren't currently impeaching him, or having him indicted through Mueller.
Hence, when polled, neither the republican constituents or democrat constituents are happy. The result: 15.1% approval and 73.6% disapproval (Real Clear Politics average of seven polls). Its equal disdain. Therefore, the mid-terms will come down to turnout.
You are certainly correct about turn-out will shape the result, that’s always important but even more so during a mid-term election. The Democrats have assumed that they would have the advantage this year with this factor, however, recent primary contests suggest that the Dem may be very wrong.
You are also correct that both parties are unhappy with the performance of their party in the Congress, but one third of the electorate are the Independents and historically they blame the party in power. It comes with the territory.
Hence, when polled, neither the republican constituents or democrat constituents are happy. The result: 15.1% approval and 73.6% disapproval (Real Clear Politics average of seven polls). Its equal disdain. Therefore, the mid-terms will come down to turnout."
In most elections Democrats will vote for the Democrat because he's a Democrat; even if they didn't live up to campaign promises.
Republicans, in general, are less forgiving.
If they believed they were misled or lied to, some Republicans will sit out the election as a protest or to 'teach 'em a lesson about breaking campaign promises'.
When that dissatisfaction level is high enough, Republicans lose.
Of course when Republicans lose, they believe they lost because they were too right-wing extreme. Hence all the center-left-moderate Republicans.
The key to Republicans winning re-election is for them to do what they promised to do.
Republicans in office never seem to get this.