There is still a primary, but the incumbent has only lost his party's nomination 4 times, only Democrat Franklin Pierce in 1856 was elected the other 3 ascended from Vice President. It has never happened since the modern primary system was created in the 1970s.
There are primaries for Dems? I thought Biden was automatically the candidate as current POTUS.
There is a whole calendar
https://ballotpedia.org/Changes_to_the_2024_Democratic_presidential_primary_calendar
There is still a primary, but the incumbent has only lost his party's nomination 4 times, only Democrat Franklin Pierce in 1856 was elected the other 3 ascended from Vice President. It has never happened since the modern primary system was created in the 1970s.