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.

<img src='https://npr.brightspotcdn.com/dims3/default/strip/false/crop/3600x2401+0+0/resize/3600x2401!/?url=http%3A%2F%2Fnpr-brightspot.s3.amazonaws.com%2F42%2Fa2%2F98c249904ddaa793f6e311f55fc3%2Fgettyimages-2268050960.jpg' alt='The U.S. Capitol is seen on March 27 in Washington, D.C.'/><p>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. </p><p>(Image credit: Samuel Corum)</p><img src='https://media.npr.org/include/images/tracking/npr-rss-pixel.png?story=nx-s1-5907379' />