Iowa Sen. Joni Ernst, who has already faced a cascade of negative advertising, saw a drop of 9 percentage points — the biggest decline in net approval for any senator. The slide places her underwater with Iowa voters (39 percent approve and 43 percent disapprove) for the first time and among the 10 most unpopular senators in the country.

Iowa voters of all partisan leanings soured on the first-term senator, but GOP voters were most likely to take a dimmer view of her job performance. Her net approval dropped by 13 points among Republicans, compared with respective 9- and 7-point drops among Democrats and independents.

Ernst is not the only Republican up for re-election next year with a home-state approval below 40 percent: Among the vulnerable incumbents, Martha McSally of Arizona, Cory Gardner of Colorado and Thom Tillis of North Carolina are all below that threshold following a quarter where each saw little movement.