You have to look district-by-district, and assess each race on its own merits. You also have to pretty much ignore the generic congressional ballot and Trump approval rating, unless it's a poll of the district you're looking at.
I actually did all this a while back. I looked at each district, examined polling in light of 2016 results, and made predictions for each race using the following objective criteria:
· Clinton won & no Incumbent running? →
Democrat
· Trump won by >5% & no Incumbent running? →
Republican
· Trump won by <5% & no Incumbent running? →
Democrat
· Clinton won & Republican Incumbent's margin >5% over Clinton's? →
Republican
· Clinton won & Republican Incumbent's margin <5% over Clinton's? →
Democrat
· Trump won by >5% & Republican Incumbent's margin ≥ to Trump's? →
Republican
· Trump won by <5% & Republican Incumbent's margin < 3% over Trump's? →
Democrat
· Trump won by <5% & Republican Incumbent's margin > 3% over Trump's? →
Republican
I generally gave the benefit of the doubt to Dems in my analysis, just to be safe. What I discovered is that Dems would need to run the table of winnable seats to eke out a majority (they can perhaps afford to lose one or two competitive races). But there is almost no margin for error. If Republicans split those legitimately competitive house races 30% to 70% in Dems' favor, Republicans would keep the House by non-negligible margin (about 5 to 7 seats IIRC). My "Dems on a good day" baseline predictions were pretty similar to
the forecasts on Inside Elections's 2018 House Midterm Elections map (202 Dem, 14 toss-up, 219 Republican).
However, if Republicans split legitimately competitive races with Dems 50 / 50, then the Republicans' margin improves to about 13 to 15 seats. That would be viewed as a major disappointment for Dems. Obviously if Republicans do better than that, it will be billed as an absolute disaster for Dems.
So, the proper question is this:
What are the chances Democrats win 95%+ of competitive House races?
I'd say Dems have about a 0% chance of that. They haven't been winning these national special elections (only 2 of 9), when Republicans were least attentive and most vulnerable. How are they going to win in places like Kansas, Kentucky, and Iowa when Republicans are getting psyched up for another "Super Tuesday" event? They're going to blow at least some of these races, and probably many more.
Realistic Probability Dems Retake House in 2018: 0.00%