With just a few primary elections to go, the competitive Senate map keeps shifting

For Democrats to take control of the U.S. Senate, they have to flip 4 seats currently held by Republicans. Republicans, meanwhile, are playing defense and pickup opportunities are harder to find.

For Democrats to take control of the U.S. Senate, they have to flip 4 seats currently held by Republicans. Republicans, meanwhile, are playing defense and pickup opportunities are harder to find.

(Image credit: Samuel Corum)