There isn't an obvious candidate snapping at Biden's heels at all, which also helps his case.
Kamala Harris has a strong base among women and black voters, but she's also not terribly well liked across the board.
Pete Buttigeg has raised his profile in the past couple of years, but his appeal to black voters is virtually nonexistant, and an openly gay President may be a bridge too far.
Probably more for 2028 than 2024, but I'd look at some of the Dem candidates who did particularly well tonight. Josh Shapiro is very much a rising star. Gavin Newsom has been very strong in California despite major opposition. With Bernie Sanders very unlikely to run again, Elizabeth Warren could also make another run as the candidate of the left, though there's a sense that her moment has passed.