Well, he's currently up in the polls, for one, and is building support with demographics that are traditionally Democrat. Biden is also the least popular President of all time at this point, which I think is a big blind spot for Dems who are saying that Trump can't win because he's so hated. Kind of ignoring the current guy's popularity, and issues he's facing right now. The Israel/Palestine shit alone could fuck him, let alone the issues he's facing on the homeland, like immigration and the economy.
Of course this is all based on polling data, which can be highly inaccurate, but it's all we got to go on, and they currently don't look very good for Biden at the moment.