Someone on the Daily Kos had a point:

However, with Donald Trump in the White House the tables are turned and Republicans are the ones who will be in a defensive position in 2018 as they now control both the White House and Congress and will have to give full account of their agenda and results to voters. In this scenario, Democrats are likely to make significant gains in the 2018 midterm elections.

Then building on that, Republicans will again be on the defensive in the 2020 general election with Trump in the White House. The core of Republican policy is set up to hurt the working and middle classes and benefit the rich and powerful. Ironically, these are the same working class people who voted for Trump. They will inevitably feel disappointed over the next four years as they realize that Trump can’t magically solve their problems.

Going into 2020 Republicans will face die-hard opposition from Democrats, a likely disillusioned white working class GOP base (the manufacturing jobs are never coming back) and an even more diverse electorate than today. All these factors will put them in a very weak position in that election. If Hillary had won this year, Democrats would be the ones on the defensive.