Republicans lose the midterm advantage that they had in 2010 and 2014. Many House seats that the Republicans won in 2010 become vulnerable if the President is at all unpopular. An R+5 seat will be vulnerable with an unpopular Republican President -- maybe someone who takes extreme positions.
Count on Democratic-friendly media turning upon the Republican President, especially if the President is seen as extreme. What Republican-leaning media did to Obama, Democrats will do to a Republican President.
To the Republicans' advantage, there just aren't many R Senate seats in play. Democrats would have to get their gains in the South, which implies reversing a 40-year trend quickly.
The Republicans really don't have a New Ronald Reagan capable of winning over 'shaky' Democrats.
I frankly think the Democrats are doomed to become an overwhelmingly urban party, meaning the House will likely be out of reach for most of the near term future. The good news is the presidency and statewide elections will become easier and easier over time, but anything that involves legislative maps is bad news for them. Winning a district by 90/10 is the same as winning one 55/45.
