Hawkeye State independents reflect the wariness of the larger electorate, with 32% pledging to back Ernst and 25% promising to vote against her, while 31% consider voting for someone else.

Trump's stumbles on trade and Ernst's unwillingness to challenge him on anything are also part of the equation. One undecided voter said she would support whoever was willing to take on Trump. "You have to get people in there who are willing to take him on,” said 62-year-old Kerri Christian.

Ernst joins a quartet of GOP senators who are facing an uphill climb to reelection: Susan Collins of Maine, Cory Gardner of Colorado, Martha McSally of Arizona, and Thom Tillis of North Carolina. In addition, Democrats just put another GOP seat in play with the announcement that Montana Gov. Steve Bullock would take on GOP Sen. Steve Daines for his seat in November.

That's half a dozen solid GOP targets to work with in a year in which Democrats need a net gain of three or four pick-ups, depending on whether Trump is reelected, in order to flip the chamber.