Like this year's national congressional races, Republicans have more to lose at the state level than Democrats. Having gained ground in three out of the last four election cycles, Republicans control both the state House and Senate chambers in 25 states; Democratic control just seven; 17 more are split.

The "blue wave" Democrats are hoping for on the national level may well extend to state houses across the country. In past midterms, the party of the president has lost around 400 state legislative seats on average, according to the National Conference of State Legislatures.

If there is a blue wave, Democrats have a solid chance to flip legislative chambers, according to Ben Wexler-Waite, communications director of Forward Majority. The super PAC is focusing on retaking legislatures for Democrats in competitive swing districts.

"The Republicans have been gerrymandering and focusing on down-ballot races for years. It's a distortion of democracy. Everyone right now is paying attention to control of Congress, but we need to start thinking about Congress two-four years from now to ensure that districts are drawn fairly," he said.

Since 1902, the incumbent president's party has lost state legislative seats in 27 of 29 midterm election cycles. (The two exceptions were 1934 and 2002.) The last major rout at the state level happened in the 2010 midterms, when Democrats got what Obama called a "shellacking." Some 708 statehouse seats changed hands, more than in any election since 1966. according to the NCSL.

Any shift in seats will have the greatest impact in states where the party in power is already holding a slim majority. That means the most critical races are being held in states where one party holds a narrow majority of seats in either chamber. Legislative chambers with single-digit percentage majorities include state senates in Colorado, Connecticut, Delaware, Maine, Nevada, New York and Washington, along with lower houses in Connecticut, Maine, New Mexico and Washington.