The campaigns do the best they can under the circumstances fate has dealt them. Their campaigns may do better or worse than expected, but the basic trends provide the context of the election contest.
Congress is the favorite whipping boy, even for members of Congress. So low approval isn't shocking. But approval at 15% is. Now as low as ever since 1992.
An opportunity to throw all the bums out. But the implications for the parties have not equally matched the views on the institution as a whole. Despite being out of power for two years, Republicans in Congress remain more negatively viewed than do Democrats, despite the latter's difficulties in passing a legislative program.
That gap between the parties, despite the overall approbation of congress, explains why the Dems look to gain despite the institutional ratings.