She could have been the next Paul Tsongas, a Democratic Senator who had run for president in 1992 and concealed his significant cancer problems from the public, and had he won, would have died in office, which would have triggered some kind of crisis.
His case is why, in modern times, Americans have always asked to know about the health of their presidential candidates.
Her death, which I do NOT hope for, (wishing her more decades of fury and angst instead), would not have caused a “crisis”. Democrats are whack-a-moles; knock one down and ten more pop up.
And this points up the importance of who their running mates are.
When LBJ became President there was no way to replace the Vice President, so if he had died before Jan. 20, 1965, the Speaker of the House would have become Acting President (or perhaps President)--the elderly John McCormack.
Had the Republican ticket won in 1940, both the President (Wendell Willkie) and the VP (Charles McNary) would have died in office before the term was ended...in the middle of WWII. The law that puts the Speaker next in line was adopted in 1947, so it may have been the Secretary of State who would have been next in line at that time.
FDR was seriously ill in 1944 but the Democrats did their best to conceal that from the public during the election campaign...but the party leadership made sure that Henry Wallace did not continue as the VP in the next term.