Anyhow, as long as Trump does a decent job (and doesn't break any of his big promises!), he will be hard to take out come 2020 as he'll be the sitting President with all the powers, prestige and advantages being an incumbent confers. And at this point I do not see anyone on the horizon for the Dims to run in 2020.
The bench for the Dims is pretty thin. They don’t have any really promising Governors out their ready to step up and make the run.
Look at the two fossils that they ran last year. Killery and Burney the Commie; those two are the bottom of the barrel.
I suspect Cory Booker will run, despite what he claims otherwise. Some dems see him as Obama 2.0...but if he did run and get the nomination, he won't have the huge historical distinction and orgasmic impact of being the potential 1st black president, esp following so soon on the heels after Obama. So what might be an enormous factor and leg up would be diminished, and some voters who went for Obama wanting to be a part of making history, won't feel the same with Booker. Also, he would be facing Trump and not spineless and scared-of-being-a-"racist" opponents like McCain and Romney. And being a confirmed, childless bachelor...not sure how that would play out on the national stage.
Many progressives loathe Booker though, for being an establishment, corporate wh0re and are enraged by his vote on the recent pharma bill. So the dems would have to deal with pissing off their Bernie wing.