A what-if on the 2024 presidential election

Could Kamala Harris have won the 2024 election if Joe Biden had resigned the presidency at the same time he relinquished the party nomination?

I saw a news item today that Mitch McConnell, who (according to Google Gemini) last cast a Senate vote on June 11, and has not appeared on Capitol Hill since the week previous to that date, is still not medically cleared to return to work after having been hospitalized and then spending a stint in a rehabilitation center.  As we've discussed previously, this long absence has led to a lot of speculation that he is worse off than he and his Senate office have admtited to - and even to some rumors that he died(!)  

It seems McConnell is still alive, if not yet completely well.  But his continuing absence may lead us to ask: is his long absence in the best interest of Kentucky and the United States?  Would they - and he - be better served by retiring?

McConnell's health struggles while in office bring to mind the sad and dramatic case of President Biden, whose visible frailties during a 2024 presidential debate may well be responsible for Donald Trump re-ascending to the presidency.  I suppose there is no need to rehash the deatils of Vice President Kamala Harris's being selected to replace him on the ballot, only to lose the election to Trump that fall.

My what-if scenario is: suppose President Biden had resigned the office of president in the summer of 2024, when his health decline could no longer be concealed from the American public.  In that case, Kamala Harris would have become president for the remainder of Biden's term.  In this scenario, it seems likely she would have been the party's candidate in the fall (bearing in mind that, had Biden resigned the presidency the same day he withdrew as the nominee, the party would have had four more weeks to try to plan and execute an orderly candidate selection process).  

But - even had Harris been the nominee in this alternative scenario, in some ways the circumstances would be very different.  She would have been running, not as a hastily and rather arbitrarily chosen replacement, but rather as an incumbent.   She would have had a chance to exhibit her executive abilities to the country.  She could have pointed to a track record of (hopefully) successfully steering the country through an extraordinary transition while maintaining calmness and normality in the economy, the markets, international relations, etc.

This alternate scenario is not a fool-proof strategy for a successor to win an election in his/her own right.  Nine times in American history, the presidency has been vacated by a death or resignation.  In all cases, the vice president filled out the remainder of the term.  All nine successors then ran for office during the following election.  But only five succeeded in getting re-elected at the top of the ticket:

President	Party	Year Vacated	Reason Vacated	Successor	Ran for Re-election?	Outcome (If Ran)
William Henry Harrison	Whig	1841	Death from pneumonia / typhoid	John Tyler	Yes	Expelled from Whig Party; ran as independent but ended campaign prior to election
Zachary Taylor	Whig	1850	Death from stomach illness	Millard Fillmore	Yes	Lost (Failed nomination)
Abraham Lincoln	National Union	1865	Assassination	Andrew Johnson	Yes	Lost (Failed nomination)
James A. Garfield	Republican	1881	Assassination	Chester A. Arthur	Yes	Lost (Failed nomination)
William McKinley	Republican	1901	Assassination	Theodore Roosevelt	Yes	Won (1904 election)
Warren G. Harding	Republican	1923	Death from heart attack	Calvin Coolidge	Yes	Won (1924 election)
Franklin D. Roosevelt	Democrat	1945	Death from brain hemorrhage	Harry S. Truman	Yes	Won (1948 election)
John F. Kennedy	Democrat	1963	Assassination	Lyndon B. Johnson	Yes	Won (1964 election)
Richard Nixon	Republican	1974	Resignation (Watergate)	Gerald Ford	Yes	Lost (1976 election)

I am not sure how much from this table would be applicable to Harris's case.  The basic trend seems to be: successors fared poorly in the 19th century, and then fared well during the 20th century, at least until Gerald Ford.  And there have been no cases since the 1970s.  It's interesting that only two Democratic presidents have vacated the office, and in both cases the vice president was subsequeently elected to serve four more years as president; and perhaps the composition of the party hasn't changed so very much since Roosevelt's coalition was put together.  

What do you think?  Would incumbency have made a difference for Harris?