Seven states hold primaries Tuesday: California, Iowa, Mississippi, Montana, New Jersey, New Mexico and South Dakota.
In Iowa, Democrats vie to take on longtime U.S. Sen. Chuck Grassley, who's likely to win his primary, and three Republicans face off to challenge Rep. Cindy Axne in the 3rd Congressional District.
Polls in Iowa close at 8 p.m. local time, 9 p.m. ET.