INDIANAPOLIS (AP) — The monthlong period for political candidates to enter Indiana's May primaries is opening, during which time the size of the Republican field for the party's U.S. Senate nomination will be determined.

The Republican race to decide the challenger to Democratic Sen. Joe Donnelly will be Indiana's top primary race. Senate candidates must submit signatures of 500 registered voters from each of Indiana's nine congressional districts to make the ballot.

Republican U.S. Reps. Luke Messer and Todd Rokita and former state Rep. Mike Braun have well-financed campaigns, while five lesser-known candidates have also filed with federal officials. The filing period opens Wednesday and ends Feb. 9.

Indiana's signature requirement nearly derailed U.S. Sen. Todd Young's 2016 campaign when a state commission kept him on the ballot despite apparently too few signatures.