VP’s are handicapped. If a VP running for the presidency in this case Harris to replace a sitting president of their own party, the VP can’t come out say what he or she would do differently or change. That would appear being very disloyal to the president you just served as VP under. Just stating that the election of the VP would be another 4 years of the previous president only worked with G.H.W. Bush in 1988. Reagan was very popular with a high overall job approval. Personalities also have a lot to do with the defeat of the VP. Nixon was kind of dour, serious. JFK filled with life. Humphrey was about the same as Nixon. 1968 had two dour candidates only LBJ in 1968 was very unpopular with a low overall job approval. Gore was a statue, no charisma although Bill Clinton was popular and charismatic. G.W. Bush gave the people that down-home boy much like Carter in 1976. Not that G.W. Bush had that much of charisma, but G.W. wasn’t a statue.
Come to think of it, most two term presidents had charisma. Certainly Obama, Bill Clinton and Reagan had it up the ying yang. All three were great orators and communicator. All three won their elections fairly easily.