On a statue in Kansas City, Missouri, an inscription reads, "David Rice Atchison, 1807-1886, President of the U.S. [for] one day." The day of President Atchison's presumed presidency occurred on March 4, 1849.
A proslavery Democrat, David Atchison served in the U.S. Senate from 1843 to 1855. His colleagues elected him president pro tempore on 13 occasions. In those days, the vice president regularly attended Senate sessions. Consequently, the Senate chose a president pro tempore to serve only during brief vice-presidential absences.
Until the 1930s, presidential and congressional terms began at noon on March 4. In 1849, that date fell on a Sunday, causing President Zachary Taylor to delay his inauguration until the next day. For some, this raised the question of who was president from noon of March 4 to noon of March 5. Of course, we now know that Taylor automatically became president on the fourth and could have begun to execute the duties of his office after taking the oath privately, a day before the public inauguration.
In 1849, the Senate president pro tempore immediately followed the vice president in line of presidential succession. That era's ever-present threat of sudden death made it essential to keep an unbroken order of succession. To ensure that there was a president pro tempore in office during adjournment periods, the vice president customarily left the Senate chamber in an annual session's final days so that the Senate could elect this constitutional officer. Accordingly, the Senate duly elected Atchison on March 2, 1849. His supporters, to the present day, claim that the expiration of the outgoing president's and vice president's terms at noon on March 4 left Atchison with clear title to the job.
Unfortunately for Atchison's shaky claim, his Senate term also expired at noon on March 4, thereby denying him the chance to become president. When the Senate of the new Congress convened the following day to allow new senators and the vice president to take the oath of office, with no president pro tempore, the secretary of the Senate called members to order.
No one planning to attend Taylor's March fifth inauguration seems to have realized that there had been a "President Atchison" in charge. Nonetheless, for the rest of his life, Atchison enjoyed polishing this story, describing his "presidency" as "the honestest administration this country ever had."