Mrs. Ernst is running against Mark Jacobs, Sam Clovis, Matt Whitaker and Scott Schaben in the June 3 Republican primary.
The winner will face off against Rep. Bruce Braley, who is fighting to keep the seat in the hands of Democrats now that Sen. Tom Harkin is set to leave at the end of his term.
The latest poll showed that Mrs. Ernst is leading the pack by a 22.5 percent to 20.4 percent over Mr. Jacobs, a former CEO of a Texas energy company.
“She’s a principled conservative, she has strong grassroots support in the state, and she can win this race if she gets her message out,” Mr. Hoskins said. “If elected, Joni Ernst will fight to cut wasteful spending, repeal Obamacare and balance the budget.”