That happens to be near an inflection point where a candidate goes from a weak frontrunner to a more formidable one. As you can see from our 2011 analysis — with a chart that is decidedly not up to current FiveThirtyEight design standards — candidates who are only polling at 20 percent despite high name recognition in the early stage of the race are often paper tigers. But get up to 30 percent, and your chances of winning the nomination improve quite a bit. That’s the point at which you may be able to win causes and primaries with a plurality; Trump won lots of states in the early going in 2016 with a vote share in the low-to-mid 30s, for example.
Biden is also fairly close to this inflection point. In general, he’s been on the happy side of it, with first-choice support in the high 20s or low 30s. But it’s possible to imagine him either gaining support (as he generates more excitement) or losing support (as he gets more scrutiny) if and when he declares for the race. There’s also a relative lack of comparatively moderate candidates in the field so far; if O’Rourke has a strong debut, it could come at Biden’s expense, for instance.