This clause that is almost always added at the end of the presidential oath taking seems to get some upset.
This is an optional phrase. It is not part of the constitutionally mandated oath of office. It is in effect a sort of prayer by the president taking the oath, a personal thing.
Article II, Section 1 of the Constitution of the United States reads 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.”
Since the inauguration of Franklin Roosevelt in 1933, all presidents have added the phrase “so help me God” after taking the oath.
The first president to use”affirm” rather than swear in the oath was Franklin Pierce in 1853. Herbert Hoover is the only other to use “affirm” in 1929. Neither gave a particular reason that I could find in my research. It is almost certain Barack Obama will use “so help me God” after his oath.