There are some safe Republican districts, just like there are safe Democratic districts. But more districts than normal in an election are now rated as competitive or leaning Democratic, and it's looking like we could see a record number of House seats flipped in this election.
I've heard some pundits say this election will not be a referendum on Donald Trump. I don't buy it. All midterm elections are a referendum on the sitting president at least to some extent -- and with Trump's unpopularity, this one is even more so. Adding to this is the complete abdication of party principles for Republicans. They have gone all in to support Trump -- even when Trump's wishes are against conservative principles (free trade, deficits, foreign policy, Russia, etc.).
The problem with this is not just that Trump is viewed negatively on his personal qualities (honesty, qualified, temperament, etc.), but as the chart above shows, the public also disagrees with Trump on most issues. Republican abdication to Trump policies means they are also out-of-step with the American public.
Trump is dragging GOP candidates down, and there is not much they can do about it at this point. They had to toss their lot with Trump to survive their primaries (because the trumpistas have taken over the party), but that will not serve them well in November.
The chart above is from the latest Economist / YouGov Poll -- done between July 29th and 31st of a nationwide sample of 1,500 adults, with a margin of error of 2.8 points.