I've happened upon this interesting blog about Abraham Lincoln. The blogger is Brian Dirck, assistant professor of history at Anderson University.
Dirck has done a series of posts on Lincoln's worst mistakes. That's an interesting angle that seems vaguely sacrilegious, at least to a northerner like me.
This one is about Lincoln's choice of a running mate (Andrew Johnson) for his reelection campaign of 1864. Some of the comments on that post discuss the question of how much a presidential candidate controlled the selection process at that point in history.
Dirck cites the two Whig vice presidents who became president upon the death of their predecessors, John Tyler and Millard Fillmore. One might think that the less-than-stellar presidencies of those two men would have caused future presidential nominees to give more thought to, and take more control over, the choice of a running mate.
I think it's an interesting commentary on human nature, that it took the McGovern/Eagleton debacle of 1972, before presidential nominees significantly changed their VP selection process. I suspect George McGovern would have lost the 1972 election anyway, even if he had not deemed it necessary to drop Thomas Eagleton from his ticket. But subsequent nominees of both parties seem to have taken a lesson from the Eagleton situation, and been much more careful in choosing running mates.
But the issue of mediocre vice presidents becoming president seems to have had far less impact on presidential candidates' behavior.
Opinions vary on the quality of the "accidental presidents" subsequent to Andrew Johnson. Theodore Roosevelt is generally listed among the greats. As to the others, my opinion is that Coolidge is underrated and Truman is overrated. And I think most of us would agree that Andrew Johnson's namesake Lyndon was a mix of good and bad, with most of us believing that the bad outweighs the good.
Be that as it may, the fact remains that, up to and including 1972, presidential nominees performed next to no due diligence on their vice presidential choices. So there was always the potential for disaster, if a low-quality vice president were suddenly called on to take over the top job.
In other words, it was only the immediate concern about possible negative impact on their candidacy, that hit their radar screens. Not that they intend to harm the country (as much as that may sometimes seem to be the case), but, when campaign decisions are made, the long-range impact on America can take a back seat, when the urgent need to obtain votes takes up all of a candidate's attention.