The GOP has a structural advantage in the Senate due to the large number of small, heavily GOP states in the Plains and West.
This will be the reason that winning the Senate will be harder and harder for Democrats since their voting population is stacked in a few states. The GOP easily wins a bunch of low populated states that get the same amount of Senators as states with much larger populations. Unless the amount of Senators changes to be based on population the Democrats will have a rough future.