John,
Voters must be convinced, and thank heavens there are undecided voters; otherwise, Democrats would have been doomed (a month ago).
Democrats have made winning the 2024 presidential election considerably more difficult because of Biden's (and his enablers') insistence on a second term and his refusal to exit the race until forced to.
Biden's vanity dragged down the Democratic chances and enabled Trump. There was little time for a selection process for the nominee; Democrats were exhausted, and Harris grabbed the opportunity.
The worry is whether or not Democrats can win the election with replacement Kamala Harris. Beyond that (that few wish to consider) is how well she would perform the job. I am unconvinced of her capacity to lead America.
I invite you to read my article "Is Kamala Harris More than Cosmetic?"
Best.