You also have to realize that historically it is less than normal for an incumbant to be defeated.
I can't help wondering if Trump will be primaried, however.
Clinton "won" in 1992, but it was after twelve years of GOP control of the WH, not four.
Jimmy Carter lost, but Carter is a Democrat and Trump is not.
The only time the GOP has lost the WH after only four years is 1892.
Democrats should be careful to predict a win in 2020. Is overconfidence ever a good thing?
The partisan label means far less than does the quality of performance. If Donald Trump performs badly as President (and Presidents now have responsibility for economic results, too), then he too can go down.
There is no obvious precedent for Donald Trump. He ran an iconoclastic campaign in which he promised to 'stick it' to millions of Americans, and I expect him to keep that promise and create a large number of resolute opponents by 2020. Will that come with improving lives for most Americans? I doubt it.
If a one-term President, then the analogies are weak for the elder Bush (incumbency fatigue will not have set in) and stronger for Hoover (got caught with a financial panic) and Carter (economic malaise. hostages in Iran). Anyone who wants to believe that the Obama recovery will continue on its own with a President who will gut the policies behind it needs far more imagination than I can supply.
I expect him to be a disaster in foreign policy because he knows nothing and will listen only to yes-men. Domestic policy? All for the Few. He wants the same sort of real-estate boom that Dubya had. Americans knows how well that worked the last time and will be chary about it. I have yet to figure, unless it is for the super-rich, for whom he will "Make America Great Again", a vapid and meaningless slogan (because of its vagueness) that can be turned on its head with Presidential incompetence.
Deporting all the illegal aliens will require a huge build-up in detention facilities... and internal passports for all Americans. There will be family break-ups because the 9-year-old is an illegal alien and the 6-year-old is a US citizen by birth. And what will be the use of those detention facilities after the 'illegal aliens' are deported? Political prisoners? No thanks!
Even more disturbing would be that America will be less attractive for people to immigrate to. Donald Trump has suggested an Education Secretary hostile to high-quality public education. Think about it. If you are an Indian with a tech degree and you have the choice between taking a job in the USA or in Great Britain and the educational system in America is gutted, then where would you go? Also consider this: some illegal aliens own businesses.
As America becomes a less-attractive place to live, expect real estate values to plummet. So you can be an engineer and your kids can be fast-food workers or retail sales clerks... maybe you might want to take your engineering degree elsewhere. There will be lots of houses with sellers but no buyers. But that means that people will be unable to use their housing equity as an ATM for the Good Life.
Pay will rise? Not if the Corporate Right gets its way and gets to end collective bargaining. Tax cuts will be strictly for the Master Class. Elite spending does not drive the economy.
One way or the other, Donald Trump will be a disaster as a President.