edit: fixed maps to reflect omissions pointed out in comments
The Cook partisan voting index ranks the 435 Congressional districts according to percent advantage for either party, e.g., R+5 or D+10 relative to presidential preference. The map above shows highlighted in yellow the 55 districts currently held by a Republican but with less than an R+5 advantage, arguably the lowest hanging fruit within striking distance in a wave election that Trump might bring.
A 55-vote swing would put Congress in Democratic control.
Below are some detail maps of the districts.