The oath or affirmation of office of the President of the United States was established in the United States Constitution and is mandatory for a President upon beginning a term of office. The wording is prescribed by the Constitution (Article II, Section 1, Clause 8), as follows:

I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect, and defend the Constitution of the United States.

"So help me God"

It is uncertain how many Presidents used a Bible or added the words "So help me God" at the end of the oath, as neither is required by law; unlike many other federal oaths which do include the phrase "So help me God." There is currently debate as to whether or not George Washington, the first president, added the phrase to his acceptance of the oath. All contemporary sources fail to mention Washington as adding a religious codicil to his acceptance.

It is important not to conflate two forms of administering, and taking, the oath of office. The first, now in disuse, is that the administrator, usually the Chief Justice, articulated the constitutional oath, requesting an affirmation, as in, "Do you George Washington solemnly swear....." At which point a response of "I do" or "I swear" completes the oath. It is reasonable to believe that this was the common procedure at least until the swearing in of Chester A. Arthur in 1881, where the New York Times article reports he responded to the question of accepting the oath with the words, "I will, so help me God."

It is the second, and current form of administration, where both the Chief Justice and the President articulate the oath, that appending "So Help Me God" is arguably a breach of the constitutional instructions. This is the contention of a Federal law suit filed in the District of Columbia by Michael Newdow on December 30, 2008. In this suit a distinction is made between the words spoken by the administrator, which must conform to the exact 35 words of the constitution, and the President, who has a right to add a personal prayer, such as "So Help Me God."

The earliest known source indicating Washington added "So help me God" to his acceptance, not to the oath, is attributed to Washington Irving, aged six at the time of the inauguration, and first appears 65 years after the event. Even if Irving's report is accurate, it would not by logic, law or intent alter the actual words of the oath, being more accurately described as a personal prayer by the President after completing the oath of office.

The only contemporary account that repeats the oath in full, a report from the French consul, Comte de Moustier, states only the constitutional oath., without reference to Washington's adding "So Help Me God" to his acceptance.

Evidence is lacking to support the claim that Presidents between Washington and Abraham Lincoln used the phrase "So help me God." A contemporaneous newspaper account of Lincoln's 1865 inauguration states that Lincoln appended the phrase "So help me God" to the oath. This newspaper report is followed by another account, provided later in the same year after Lincoln's death (April 15, 1865), that Lincoln said "So help me God" during his oath. The evidence pertaining to the 1865 inauguration is much stronger than that pertaining to Lincoln's 1861 use of the phrase. Several sources claim that Lincoln said "So help me God" at his 1861 inauguration, yet these sources were not contemporaneous to the event. Shortly after giving the speech, Lincoln stated that his oath was "registered in Heaven.", something some have taken as indicating he likely uttered the phrase "So help me God." Conversely, there was a claim made by A.M. Milligan (a radical Presbyterian minister who wanted the U.S. government to be officially Christian) that letters were sent to Abraham Lincoln asking him to swear to God during his inaugurations, and Lincoln allegedly wrote back saying that "God's name was not in the Constitution, and he could not depart from the letter of that instrument."

Other than the president of the U.S., many politicians (including Jefferson Davis, sworn in as president of the Confederate States of America in 1861) used the phrase "So help me God" when taking their oaths. Likewise, all federal judges and executive officers were required as early as 1789 by statute to include the phrase unless they affirmed, in which case the phrase must be omitted.

Since FDR, all presidents have added the phrase "So help me God."

Ancillary practices

The oath or affirmation is typically administered by the Chief Justice, but sometimes by another federal or state judge (George Washington was first sworn in by Robert Livingston, the chancellor of the State of New York in 1789, while Calvin Coolidge was first sworn in by his father, a Vermont notary public, in 1923). By convention, incoming Presidents raise their right hand and hold the other on a Bible or other book while taking the oath of office.

Franklin Pierce was the first president to use the word affirm rather than swear. Theodore Roosevelt did not use a Bible when taking the oath in 1901. Dwight Eisenhower, Harry Truman, and Richard Nixon swore the oath on two Bibles. John Quincy Adams swore on a book of law. Lyndon B. Johnson was sworn in on a Roman Catholic missal on Air Force One. Washington kissed the Bible afterwards, as some later Presidents did, but modern Presidents have not—except for Harry Truman, who bent and kissed the Bible upon taking the oath for the first time, on April 12, 1945, as well as at his second inauguration. Many times the President-elect's name is added after the "I"; for example, "I, George Washington, do . . ." Lyndon B. Johnson did not add his name when swearing his first oath of office; there is evidence that in all other inaugurations since Franklin Roosevelt's first, the name of the president was added to the oath. William R. King is the only president or vice president sworn into office on foreign soil. By special act of Congress, he was allowed to take his oath on March 24, 1853, in Cuba, where he had gone because of his poor health. He died 25 days later. Sarah T. Hughes is the only woman to administer the oath of office (she was a U.S. District Court judge who swore Lyndon Johnson into office on Air Force One after the Kennedy assassination).