Fifteen states are sending a one-party delegation to the new 115th Congress -- meaning all of their House representatives and both of their senators are from the same political party. As the top chart shows, nine states are sending wholly Republican delegations, and six states are sending wholly Democratic delegations. The other 35 states are sending delegations that are a mix of both political parties.
The second chart shows the historical incidence of states sending one-party delegations. Note that the current fifteen states is the largest number of one-party delegations since 1957 -- sixty years ago. This seems to be a growing trend. There were seven states in the 112th Congress, nine states in the 113th Congress, thirteen states in the 114th Congress, and now fifteen states in the 115th Congress.
The chart below shows the number of times each state has sent a one-party delegation to Congress (if ever), and the number of times for each party in that state.