Realistically, unless 2016 ends up being a Democratic tsunami, Democrats will not control the Senate in 2019.
This. As much as I like Heitkamp, McCaskill, Donnelly, and Tester, I realize that they likely won't survive 2018. And if the republicans can find someone competent to challenge Brown in OH, he'll be gone as well. And then there's Nelson, Kaine, Casey, Baldwin, Stabenow, Cantwell, and Heinrich at various levels of vulnerability. If Klobuchar retires, her seat will have some vulnerability as well.
For the democrats, Nevada is the only realistic pickup. Arizona is not realistic in a midterm unless Sinema runs.