After a contested primary, once someone has secured the nomination of the party, they then get to appoint the head of the national political committee (DNC/RNC). Those orgs then functionally become extensions of that person's campaign. There used to be more separation but over the past 20 years shifts in campaign finance law (much of which has been driven by SCOTUS rulings) has really reduced the power of the national parties as institutions. It's led to some problems where we have high polarization but weak parties, making it easier for extreme or unconventional candidates who appeal to various subsections of the parties to get traction.
Thank you for answering the main questions that's been on my mind of "why the hell has the DNC put Biden up again for re-election"!
After a contested primary, once someone has secured the nomination of the party, they then get to appoint the head of the national political committee (DNC/RNC). Those orgs then functionally become extensions of that person's campaign. There used to be more separation but over the past 20 years shifts in campaign finance law (much of which has been driven by SCOTUS rulings) has really reduced the power of the national parties as institutions. It's led to some problems where we have high polarization but weak parties, making it easier for extreme or unconventional candidates who appeal to various subsections of the parties to get traction.