In short, while it might seem a bit weird that presidential approval ratings and the generic ballot have moved in opposite directions, the data isn’t that hard to explain. The president’s party usually does poorly at the midterms even with a popular president, and Trump isn’t popular. His numbers are improved, but only marginally. Moreover, the relationship between presidential approval and midterm performance is rough enough that you wouldn’t necessarily expect them to move in lockstep with one another. And Republicans aren’t doing any worse (or better) than you’d expect from historical trends. Maybe the Republican outlook in the House would be even worse without the recent uptick in Trump’s approval rating. But that outlook isn’t good, and while Trump is probably still a net liability for the GOP, Republicans have plenty of problems of their own making too.