Forget custom. The 20th Amendment states that the new presidential term begins at noon on January 20. Not when the president-elect takes the oath of office. The oath allows the new president to execute the office, as the Constitution says. But he has become president because a) the Senate has so certified his election, and b) the stroke of noon, Jan. 20 has passed.
Thanks, everyone, for submitting so many interesting- and varying replies. I suspect the topic is one which could still be argued about for years to come (especially if you’re ‘in’ to the history of the USA Presidents, as I am.
And, thanks, Zapper, for your welcome to the SDMB. I have this nasty feeling it’s going to be taking up a lot of my spare time, because it’s so fascinating.
Besides the other dozen or so solid reasons given for why David Rice Atchison was not president for a day, here is another: The United States Constitution.
Article I, Section 6:
“[N]o person holding any office under the United States, shall be a member of either House during his continuance in office.”
David Rice Atchison was a member of the U.S. Senate. Unless he formally resigned from the Senate, he was ineligible to hold another federal office, including the presidency.