The poll shows Grassley close to the 50 percent approval rating line, under which incumbents are sometimes considered vulnerable. Still, he maintains a healthy margin between his approval and disapproval numbers.

The poll didn't test Grassley against a generic or specific Democratic candidate, either, though Iowans said they were split over whether they would prefer to keep or replace their incumbent candidates for federal office this fall. 45 percent said they would prefer to keep their incumbents in place, while 45 percent said they would prefer to see someone else in office.

By contrast, Grassley's Iowa colleague, Sen. Tom HarkinThomas (Tom) Richard HarkinBottom line Trump's trial a major test for McConnell, Schumer New Hampshire parochialism, not whiteness, bedevils Democrats MORE (D), faces similar -- though slightly more negative -- approval numbers.

51 percent of Iowans say they approve of the way Harkin, the chairman of the powerful Senate Health, Education, Labor and Pensions (HELP) Committee, is handling his job. 34 percent said they disapprove of Harkin's job performance, while 15 percent are undecided.

Harkin's approval, like Grassley's, took a slight tumble. A November Iowa poll showed he enjoyed a 54 percent approval rating.

Grassley is up for reelection this fall, though no top-tier Democratic candidates have emerged to challenge the senator. Harkin just won reelection with 63 percent of the vote in 2008, and is not up for reelection until 2014.