Not too many Senate opportunities for the GOP in the 2020 cycle. Only Alabama (fluke Dem win), Michigan (Black GOPer John James is nearly tied with the freshman Dem incumbent Peters in a recent poll) and New Hampshire (Shaheen stands a good chance to lose against Gov. Chris Sununu) are the best chances for pickups. This presumes no losses, and the GOP is endangered in AZ (McSally, who lost last year), CO (Gardner) and potentially NC (with the flaky Tillis).
Which is exactly why the impact of the BOGUS Russia Investigation not only affected the 2018 Midterms but could impact the GOP Senate Majority in 2020.