The 2018 midterms will likely be bad for the GOP despite the good map as Trump's unpopularity will cancel it out. He is already at 40% approval and this is his honeymoon, after two years of his disastrous 'leadership' it will probably be lower and the Democrats will do well(not sure if they can gain either house though). Could go differently but this us the most likely scenario.
I doubt he has a 40% approval rating among the midterm electorate in states like Missouri, Indiana, Montana, North Dakota, West Virginia, Ohio. Maybe he has a 40% approval rating with the whole size of the American people, but only a third will go to the polls in a midterm and having an approval rating of 10% in California, Massachusetts or Illinois won't be a problem in terms of Senate control.
In 2010 the tea party wave won a senate seat in Massachusetts where Obama was popular in 2006 the dems won house seats in deep red areas were Bush was still popular when there is a mood in the country that always causes a swing
