When the Deep State of the time went after Nixon, they went for Vice President Spiro Agnew FIRST, to assure they would not have to go through the same thing twice.
Agnew had problems of his own stemming from his political career in Maryland, before he got to the VP slot. The threat was that whatever they had on him would be exposed and prosecution would proceed, unless he left quietly and right now, in the dark of the night. Agnew resigned and left the VP slot vacant, allowing Gerald Ford to be named VP, and after the Nixon resignation, Ford became the only man to ever ascend to the Presidency without ever being elected to the office of President or Vice President first.
The Democrats want to preserve that symmetry, only with their own choice for the appointment.
During the Agnew-Nixon lynching, there was no constitutional way to appoint a new VP. That was legislated specifically after the fiasco.