I remember seeing the movie version of “The Man” back in the 1970’s.
The plot of the movie was a bit different. The President and the Speaker of the House were killed when a building collapsed on them while visiting Germany.
The Vide President was a man who was in a wheel chair and very sick, and he refused to take the oath of office. “We just had the funeral of one president; we don’t need the funeral of another.”
Next in line....the President Pro-tem of the Senate, a black guy (played, I think, by James Earl Jones).
Burgess Meredith was in the movie, and he said he wouldn’t be surprised if a bunch of southern states seceded.
It was so long ago. Yes, I think you are right. He was President pro temp.