CharlieHall: You’re a terrific virologist but (wadr) a poor constitutional scholar. There is a powerful argument that the Succession Act’s placement of the speaker in the line of presidential succession (and after her, the president pro tempore of the Senate) is contrary to the Constitution’s Succession Clause. That clause states that only an “Officer” may succeed and act as president. Most of the pertinent commentary maintains that the term “Officer” here does not include members of Congress. As such, Pelosi could not constitutionally “act as President,” even though the statute says she can.
Secretary of State Mike Pompeo (the executive officer next in line under the statute) can declare himself acting president on the basis of a legal opinion from the Attorney General proclaiming legislative succession to the presidency unconstitutional.
Furthermore, as I mentioned, to preclude even getting to the above scenario (that I described in my previous comment) the Senate before January 20 could have a contingency election for Vice President and elect Michael Pence. Then Pence would remain vice president after Trump’ term ended at noon January 20 and Pence would precede all other persons in assuming the presidency.