Absent some catastrophic event, the Dems aren't going to take the Senate in 2018. The map for them is horrible this cycle.
Trump is not going to be impeached unless there's incontrovertible proof of collusion between his campaign and the Russian government. I think the odds of that are low. My guess is that there's going to be circumstantial evidence and a lot of stink, but no smoking gun.
There are all kinds of other little stinkbombs floating around out there (the New Yorker's Trump Baku article this week detailing TO deal with a massively corrupt Azerbaijani minister wired into the Iranian Revolutionary Guard is a good example) but with Republicans running oversight, they're never going to amount to much. Trump is unpopular for a newly elected President, but Democrats, generically, are even less popular.
I think the Trump claims of 4%+ growth are nonsense, but I agree with thethe that his prospects are going to be tied to the economy. If the economy is humming in 2020, there's every chance he'll be re-elected. If it's not, his act is going to wear thin.