An article recently came out (well it would have to be recent) detailing everything that happened after Biden informed her that he would step aside. She assembled her team, informed them, and began working the phones calling each of her potential challengers, supporters, and key democratic constituencies one minute after Biden informed his staff and personally made 100 cals in 10 hours.
This is an incredibly disciplined person and although it was an unprecedented situation she and the dem leadership and the rank and file have absolutely made it work. She is the nominee for all intents and purposes.
While a lot of voters vote the person and not the party its a two party system in which for anything to get done the party has to sweep both the executive and legislative bodies (Presidency, House, and Senate). In addition these days if your party doesn’t control the senate you lose the Supreme Court and your legislation will be blocked or overturned.
I am a yellow dog democrat—that is to say I vote for the party regardless of standard bearer. Because, more or less, the democrats offer the best chance to further my policy preferences. But lots of people won’t get out of bed to vote unless they love the candidate. Specifically lots of voters focus primarily on the presidency and are unaware of the other seats being fought or the other issues in power/legislating. This is why both parties try to put attractive ballot issues on the ballot if they can. For Republicans its usually something punitive or “patriotic” like anti abortion, anti immigrant, or pro flag ballot initiatives. For the dems it used to be things like pro-pot legislation. That brought out socially liberal but otherwise laxy voters who might then pull the lever for the dem candidate.
The presidency serves the same function: an exciting or charismatic candidate can pull unlikely voters to the booth or energize likely voters. An uncharismatic or damaged one can fail to get people out to vote at all so the downballot races are affected.
I agree with the “brave tell it like it is” poster that overconfidence could be bad. But misery and nihilism are really just as dangerous as they discourage voters from voting as they won’t put effort into what they see as a losing cause.
The dems have a narrow needle to thread here with respect to the Senate regardless of candidate. It will be nearly impossible to take the Senate. But still the charisma and the ground game of the presidential nominee matters because all those enthusiastic fundraisers and door knockers will also be turning out the senate and congressional vote.
In conclusion: enthusiasm matters.