Sprague was elected a U.S. Senator in 1863. During the 1868 impeachment trial of President Andrew Johnson, presided over by Chief Justice Salmon Chase, Sprague kept his intentions to himself, but ended up voting with most Republican senators for conviction. This may have furthered his rift with Kate, whose father's chances for the 1868 Republican Presidential nomination would have been damaged had Johnson been removed from office. Next in line to the Presidency, under the law at the time, was radical Republican President pro tempore of the U.S. Senate Benjamin Wade, who could have then run as an incumbent. Johnson was acquitted by a single vote.