On February 28, 1844, during a pleasure cruise/exhibition on the Potomac River, the U.S. Navy's first propeller-driven steamship, U.S.S. Princeton, made to fire two of her guns for the benefit of visiting dignitaries. Sadly, the intended gesture went foul when one of the guns exploded, killing Secretary of State Abel P. Upshur, Secretary of the Navy Thomas Gilmer, and several others. President John Tyler was very nearly also killed, but was saved because of a brief stop below decks.
What if he hadn't stopped? What if he had gone directly to the main deck and had been killed in the explosion?
Because he had succeeded to office upon the death of President W. H. Harrison and U.S. law of the period did not allow for the selection of a new Vice-President until the next general election, Tyler's next in line would have been Willie Person Mangum, who was then serving as President pro tempore of the U.S. Senate.
Now, assuming nothing happens to keep him from assuming office, how does President Mangum act differently from President Tyler? What different challenges does he face while in office? Does he try to seek a term of his own in the White House, and if so, who does he pick as his running mate and what are his chances?