Who do they want if Biden does step down?

[DemocraticPatriot]

78. It's only a DNC committee task if it happens AFTER the convention...

If she were nominated at the convention, she would be able to choose her own running mate, as usual-- presuming that the convention delegates would vote for her choice, as usual, in the Vice Presidential nomination balloting.

(Well, actually she could name one before being nominated officially-- there's nothing to prevent that.)

If either the presidential or vice presidential slot became vacant after the convention,
then the members of the Democratic National Committee would choose a replacement.

(The Vice Presidential nominee does not become the presidential nominee, unless the DNC voted as such).

However, if the presidential candidate was elected, and died between the election and the inauguration,
the Vice-President elect would be sworn in as president.

(We had long discussions about all this a few months back, and looked to the party rules about it.)