Sen. David Vitter (R-La.) has attracted 16 primary challengers, including a former Louisiana Supreme Court justice.
Republican Chet Traylor, who left the court in 2009, was the last candidate to register before the qualifying period ended Friday afternoon.
Traylor was encouraged to run by Republicans who are dissatisfied with Vitter, according to the Associated Press. The first-term senator, already tainted by a 2007 prostitution scandal, has recently been hounded by questions about his former aide's arrest for assaulting his girlfriend with a knife.
State Rep. Ernest Wooton entered the race as an Independent. He will bypass the August primary and appear on the November general election ballot.