Iowa Agriculture Secretary Bill Northey (R) will not run for the Senate, he announced on Twitter Thursday, saying he hopes Rep. Steve King (R) will enter the race.

King is the big question mark on the GOP side. The outspoken conservative has been mulling a bid for months. He told the Des Moines Register on Wednesday that he's "embarrassed" he hasn't reached a decision yet.

Aside from Northey, Rep. Tom Latham and Lt. Gov. Kim Reynolds have ruled themselves out of the Republican race. Former U.S. attorney Matt Whitaker, state Sen. Brad Zaun and Secretary of State Matt Schultz are possible Republican candidates.

On the Democratic side of the race for retiring Democratic Sen. Tom Harkin's seat, Rep. Bruce Braley (D) is the likely nominee.