Actually, the GOP has a decent chance to keep the Senate in 2018 based on the seats up for grabs in two years.
The Senate election in 2018 will have 25 Democrat seats (including 2 independents who caucus with Dems) and 8 GOP seats up for election. The 8 GP seats include "safe" seats in Mississippi, Nebraska, Tennessee, Texas, Utah, and Wyoming.
The Democrats have to defend seats in Florida, Indiana, Michigan, Missouri, Montana, North Dakota, Ohio, Pennsylvania, West Virginia, and Wisconsin...states that all just went for Trump last night. If everything goes right for Trump and the GOP the next two years, they could emerge in 2019 with 60+ seats in the Senate.