I like K.I.S.S. methods, and I have decided that the easiest K.I.S.S. guide to the midterm elections is likely to be Rasmussen's "net strong disapproval" spread.  The theory is that while voters who even weakly approve or disapprove of a President are likely to come out and vote in the Presidential election years, only those with strong opinion -- a substantially smaller number -- come out to vote in midterm elections.

Here's what Rasmussen's net disapproval and net strong disapproval looked like during the Obama years:

Obama had a 1:1 approval vs. disapproval spread on Election Day 2012 (vertical red line), and managed to win re-election.

But on Election Days 2010 and 2014, for every 100 adults who strongly disapproved of Obama, there were only 60-65 and 55 adults who strongly approved of his performance -- enough for a GOP wave in each case.

Over the last few months, Trump's net strong disapproval ratings have gotten progressively worse:

And today, for the first time, the ratio of strong disapprovers to strong approvers hit 2:1:

Worse for the GOP, the 50%  strong disapproval means that if those people come out to vote, literally the GOP cannot win.

Rasmussen's index is only a nowcast, not a forecast, but if the polling 15 months from now is as bad as it is today, the GOP's only hope of not losing its House majority is its ruthless gerrymandering and voter suppression.