David Rice Atchison (August 11, 1807January 26, 1886) was a mid-19th century Democratic United States Senator from Missouri. He served as President pro tempore of the United States Senate for six years. He is best known for the claim that for one day (March 4, 1849) he may have been Acting President of the United States. This belief, however, is dismissed by nearly all historians, scholars, and biographers.