Political analyst, Larry Sabato says that democrats face a blood bath in 2018.  He noted that in 2016, the democrats were expected to win the White House, take control of the Senate and cut into the republican lead in the House by a considerable margin.  However, Trump won the presidency, democrats only picked up a couple of seats in the Senate (Both NeverTrumpers) and only picked up 4 to 7 seats in the house.  But he notes that 2018 is a whole new ballgame and republicans could greatly increase their leads in all areas.

Before 2018 comes around, the democrats could lose two senators.  Joe Manchin is said to be considering a switch in parties but he could also be picked by Trump to join his administration.  West Virginia is now a heavily republican state and chances are they would select a republican to take his place.  Trump is also looking at Heidi Heitkamp for his administration.  The republicans could go into 2018 with a 54-46 lead in the Senate.

In 2018 there are 8 republican senators up for reelection and all in deep red states.  The democrats on the other hand will have to defend 25 seats.  Ten of those Senators are in states carried by Donald Trump.

Sabato points out:

The last time a party was as exposed as Democrats are in 2018 was in the 1970 cycle. At the end of 1968, 25 Democratic-held seats were up in the 1970 midterm. There are some similarities between the position of Democrats in 1970 and 2018. First, Class I Senate seats were up in 1970, just as they are in 2018. Second, a sizable number of Democratic-held 1970 Senate seats (13) were up in states that Republican Richard Nixon had just carried in the closely-contested 1968 presidential election, compared to the 10 Democrats are defending in 2018 in Trump states. Perhaps endangered Democrats up in 2018 can feel a little bolstered by the fact that Democrats only lost three net seats in the 1970 midterm despite having to defend numerous seats, many in states that backed the most recent GOP presidential nominee. Overall, 11 of the 12 Democratic incumbents running in states won by Nixon in 1968 won reelection in 1970 (though Harry Byrd Jr. of Virginia ran as an independent that cycle, eschewing his previous party label).

One point Sabato failed to touch on is that voters have suffered through 8 years of Obama and a successful two years of Trump or democrats blocking popular legislation could magnify them problems the democrats have.  Donald Trump could easily face 2 years of a filibuster proof Senate.