If none of the Republicans running for Senate in Iowa clears 35 percent in a primary, the nominee will be chosen at a mid-June convention. There is concern that activists loyal to the Ron Paul wing of the party, perhaps in a coalition with social conservatives, could pick someone without the resources to win in November.

Democrats quickly cleared the field for Rep. Bruce Braley after Democratic Sen. Tom Harkin announced his retirement early this year.

The two frontrunners for the GOP nomination are seen as state Sen. Joni Ernst and former business executive Mark Jacobs, who has the ability to self-fund. But the crowded field also includes former U.S. Attorney Matt Whitaker, talk radio host Sam Clovis, and former Chuck Grassley aide David Young.

Sen. Bob Bennett (R-Utah) was toppled at a 2010 convention, but the GOP easily held the seat because it is the country’s most Republican state.

Iowa is a purple state and presidential battleground, as are two other states where the GOP nominee may be selected by convention, Virginia and Minnesota.