It is an old favorite among trivia question writers. "Who was the only former American president to serve in the United States Senate?" The answer is identical to that for another popular civics question: "Who was the first president to be impeached in the House and tried by the Senate?"

On July 31, 1875, Senator Andrew Johnson died of a stroke while visiting his daughter in Tennessee. The Tennessee Democrat had first served in the Senate from 1857 to 1862. In the early months of the Civil War, Johnson—the only southern senator to remain loyal to the Union after his state seceded—was obliged to flee that state to avoid arrest. When federal troops conquered Nashville, he resigned his Senate seat in March 1862 to accept President Abraham Lincoln's appointment as military governor of Tennessee. In 1864 he won election as vice president and served for a month in 1865, until moving to the White House to serve as president for the balance of the assassinated Lincoln's term.

In January 1875, Johnson won back his former Senate seat after a hotly contested struggle that forced the Tennessee legislature through 56 separate ballots. On March 5, 1875, Johnson took his Senate oath before the same body that only seven years earlier had failed by a single vote to remove him from the presidency. During the 19-day Senate special session, he delivered one major address—on political turmoil in Louisiana—and then returned to Tennessee, where he died four months later.

In later years and without much enthusiasm, the Senate periodically considered proposals permitting former presidents to attend Senate sessions, either as at-large members or in some advisory capacity. Finally, in 1963 the Senate adopted Senator Claiborne Pell's amendment to Rule XIX allowing former presidents to address the Senate "upon formal written notice to the Presiding Officer." Although several ex-presidents have stopped by to say hello, none has yet chosen to make a formal address.