The contests are close in Arizona, Georgia, Nevada and Pennsylvania. Many voters want Republicans to flip the Senate, but prefer the Democrat in their state.
Widely considered the most endangered Democratic incumbent in the nation, Senator Catherine Cortez Masto is battling for survival in a race that may determine the Senate majority.
While Democrats are optimistic about holding the Senate, and Republican campaigns have faced a huge financial disadvantage, races are tightening across the country as the November election approaches.