Outgoing President James Polk's term expired at midnight, March 3/4, and his successor, Zachary Taylor, refused to be sworn into office on the sabbath (Sunday). Taylor's Vice Presidential running mate, Millard Fillmore, likewise was not inaugurated. As President ProTempore, under the Presidential Succession Act of 1792, Atchison was believed by some to be Acting President.
Atchison never took the oath of office, although there was no constitutional or legal requirement, for an Acting President to do so. Though Article II Section 1 Clause 8 mandates that the president must take the oath before "...he enter on the Execution of his Office.." so Taylor was not president . Fillmore was not constitutionally mandated to take the oath of office to be Vice President, though it is currently prescribed by statute that he take an oath. So while Atchison might have been Acting President, Filmore might also have been a possible Acting President if there wasn't a law mandating a Vice Presidential oath at the time. Or for 36 hours the US might not have had a President. Which would have been the cause for 12 of those any way.
Atchison discussed the claim in a September 1872 issue of the Plattsburg Lever:
- It was in this way: Polk went out of office on the 3d of March 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.
Atchison was 41 years and 6 months old at the time of his "presidency", which, if it had been official, would still make him the youngest President in American history. Theodore Roosevelt, the youngest to serve, was 42 years and 11 months old when he was sworn in following the death of William McKinley in 1901, and John F. Kennedy, the youngest to be elected, was 43 years and 7 months old when he was inaugurated in 1961.