NBC News has identified the four most important states to watch in 2018:

Arizona: Competitive Senate contest that could decide control of the chamber; emerging presidential battleground state; a couple of competitive House races (AZ-1, AZ-2); large Latino population.

Florida: Competitive gubernatorial race (both in the primaries and general) and whoever wins could be a national star; likely competitive Senate race; huge 2020 battleground; and a handful of must-watch House races (FL-27, FL-26, FL-18).

Ohio: Competitive gubernatorial race (both in the primaries and general) and whoever wins could be a national star; likely competitive Senate; huge 2020 battleground.

California: As many as NINE competitive House races (CA-7, CA-39, CA-49, CA-25, CA-48, CA-10, CA-45, CA-21, CA-50). If Democrats win six or more of these districts, they’re likely going to win back control of the House.