Democrats have a better chance of regaining control of the US Senate in 2020.

In 2018, Democrats will hold onto every Democratic held US Senate seat but IN(Donnelly-D)and ND(Heitkamp-D). Donnelly-IN and Heitkamp-ND face top tier GOP challengers. Democrats will end up picking up AZ (Sinema-D) and NV (Titus-D).
In 2020, Democrats will hold onto every Democratic held US Senate seat including MI (Peters-D),MN (Franken-D),NH (Shaheen-D),and VA (Warner-D) and pick up CO (Polis-D),GA (Ossoff-D),NC (Ross-D) and TX (Castro-D) if Cornyn-R retires. Democrats will also win the 2020 United States Presidential Election.