They'll also decide nominees for all four of the state's U.S. House seats and one of its two Senate seats.
Presidential candidates Newt Gingrich, Mitt Romney and Rick Santorum have all campaigned several times this past week in Mississippi and neighboring Alabama, which also holds its primary Tuesday.
Mississippi has 40 Republican delegates, and 37 of them are awarded in the primary.
Two Republican freshman congressmen, Alan Nunnelee and Steven Palazzo, face challengers from the tea party faction of the GOP.
Longtime Democratic Rep. Bennie Thompson is challenged by former Greenville Mayor Heather McTeer.
Republican Rep. Gregg Harper faces one primary challenger, and Republican Sen. Roger Wicker faces two.
Three are seeking the Democratic nomination for Senate.
Visit DJournal.com for election results Tuesday.