President Trump won the election because he spoke to a large group of people in this country who felt left behind, many of whom would have been traditional democrat voters. He'll get elected again unless the DNC can figure out howto plug that hole, because as long those people perceive him as being in their corner, they'll be out voting for him, bringing in states like Michigan, Wisconsin, and Pennsylvania.
There are more factors than just one. But that he got that close had a lot to do with that factor. Didn't hurt that 12% of Bernie Bros voted for Trump. Or that Hillary didn't campaign in the swing states which he won. Will he win again? He could. Midterms went a bit worse for Republicans than was estimable from historical elections. Not sure I'd go so far as to call it a blue wave, but it seems clear that a large voting block came out to voice their displeasure with Trump. I think we have to see who runs against Trump to know if the same people who voted the first time will vote for him again. I think the group you're talking about who Trump spoke to is maybe 1/3 of the electorate. So Trump needed some help from other places to win.