So 5 years right?
I can't really see a scenario where the Democratic candidate for President wins in 2020 without having a Democratic Senate majority, unless the GOP stalls the blue wave this year.
Probably the only seat I could see the Democrats losing in 2020, assuming their Presidential candidate wins, is Alabama, and I could see them picking up Colorado and Maine at least. (Though that latter really depends on Collins' future career plans. There was some talk that she'd make a gubernatorial run this year but obviously that didn't happen.) Kentucky's a possibility, mainly because of how incredibly unpopular Mitch McConnell is.