Only South Dakota voters who have already registered can vote in the June 3 primary.
Voter registration has ended, 15 days before the election as required by state law.
The most recent figures show nearly 511,000 people registered to vote in the state. Forty six percent registered as Republicans and 34 percent as Democrats. Independents make up 16 percent.
Registered voters can cast their ballots early with absentee voting by mail or at county courthouses. Officials have sent out over 4,500 ballots and received nearly 3,300 back.
In the June primary candidates for governor and U.S. Senate will face challengers from within their party, as will contenders for state House and Senate.
Registered voters must bring photo identification to the polls or sign a personal identification affidavit.