WIKI:
David Rice Atchison (August 11, 1807 – January 26, 1886) was a mid-19th-century Democratic[1] United States Senator from Missouri.[1] He served as president pro tempore of the United States Senate for six years.[2] Atchison served as a major general in the Missouri State Militia in 1838 during Missouri's Mormon War and as a Confederate brigadier general during the American Civil War under Major General Sterling Price in the Missouri Home Guard. Some of Atchison's associates claimed that for 24 hours—Sunday, March 4, 1849, through noon on Monday—he may have been acting president of the United States. This belief, however, is dismissed by most scholars.[2][3]

𝐏𝐮𝐫𝐩𝐨𝐫𝐭𝐞𝐝 𝐨𝐧𝐞-𝐝𝐚𝐲 𝐩𝐫𝐞𝐬𝐢𝐝𝐞𝐧𝐜𝐲
Inauguration Day—March 4—fell on a Sunday in 1849, and so president-elect Zachary Taylor did not take the presidential oath of office until the next day out of religious concerns. Even so, the term of the outgoing president, James K. Polk, ended at noon on March 4. On March 2, outgoing vice president George M. Dallas relinquished his position as president of the Senate. Congress had previously chosen Atchison as president pro tempore. In 1849, according to the Presidential Succession Act of 1792, the Senate president pro tempore immediately followed the vice president in the presidential line of succession. As Dallas's term also ended at noon on the 4th, and as neither Taylor nor vice president-elect Millard Fillmore had been sworn into office on that day, it was claimed by some of Atchison's friends and colleagues that from March 4–5, 1849, Atchison was acting president of the United States.[21][22]

Historians, constitutional scholars, and biographers dismiss the claim. They point out that Atchison's Senate term had also ended on March 4.[3] When the Senate of the new Congress convened on March 5 to allow new senators and the new vice president to take the oath of office, the secretary of the Senate called members to order, as the Senate had no president pro tempore.[21] Although an incoming president must take the oath of office before any official acts, the prevailing view is that presidential succession does not depend on the oath.[3] Even supposing that an oath was necessary, Atchison never took it, so he was no more the president than Taylor.[3]

In September 1872, Atchison, who never himself claimed that he was technically president,[3] told a reporter for the Plattsburg Lever:

It was in this way: Polk went out of office on March 3, 1849, on Saturday at 12 noon. The next day, the 4th, occurring on Sunday, Gen. Taylor was not inaugurated. He was not inaugurated till Monday, the 5th, at 12 noon. It was then canvassed among Senators whether there was an interregnum (a time during which a country lacks a government). It was plain that there was either an interregnum or I was the President of the United States being chairman of the Senate, having succeeded Judge Mangum of North Carolina. The judge waked me up at 3 o'clock in the morning and said jocularly that as I was President of the United States he wanted me to appoint him as secretary of state. I made no pretense to the office, but if I was entitled in it I had one boast to make, that not a woman or a child shed a tear on account of my removing any one from office during my incumbency of the place. A great many such questions are liable to arise under our form of government.[23]