Answer
Five U.S. senators have served longer than Grassley. These senators are Patrick Leahy (D-VT), Dianne Feinstein (D-CA), Richard Shelby (R-AL), John McCain (R-AZ), and Orrin Hatch (R-UT). Leahy has served since 1975, Feinstein since 1992, Shelby since 1987, McCain since 1987, and Hatch since 1977. Grassley has served since 1981, making him the sixth-longest-serving current senator.