In the 2016 election, Republicans won the national House popular vote 49-48. In that same election, despite winning the electoral college, Donald Trump lost the national popular vote by 2 points (48-46). So, the GOP House candidates ran 3 points better than Trump.
Trump has been a controversial president (I don't think it is partisan to say that) and I hardly think it is unfounded to suggest that there are people who disapprove of his presidency who would still vote for a GOP rep in a congressional race. Conversely, I doubt there are many folks at all who approve of the Trump presidency who will vote for a democrat in their House race. Put simply -- if you like Trump, you probably really like the GOP. If you dislike Trump, it does not mean you hate the rest of the GOP (I have numerous GOP friends who fall into this category).
So, if Trump gets to only a minus 6 or 7 on his approve/disapprove number, it could easily translate to the GOP being fairly close to break even in the national House popular vote. It is generally accepted that the Dems need to win the nationwide House vote by a couple points to take control of the House thanks to the Gerrymandering that has carved out extra GOP seats in many states. Finally, it is worth noting that presidential approval numbers are polls done of all American adults, not likely voters. As we all know, turnout and passion are a huge deal in mid-term elections and the GOP has a history of being better at turnout than Dems in mid-terms (partially owing to the older nature of the GOP base and the fact that older folks vote at a much much higher rate than young people).
So, if Trump can get to minus-6 or 7, I think the GOP's chance of holding the House will be around 50%, maybe even higher.
-Jason "according to 538, the GOP currently has a 25% chance to hold the House... if the Pres is 4 points more popular how can anyone think the GOP odds will not go up significantly?" Evans