The argument for Biden is easily stated; he is the candidate with the best chance to beat Donald Trump. So far, it has worked reasonably well; in spite of his obvious weaknesses, Biden still leads in most national polls. It isn’t very inspiring, however, and it puts him at the mercy of the polls. It also doesn’t provide any guidance as to how he should behave once in office. Is something better available?
Yes–he can sell himself as the candidate who is best positioned to bring the country back together.
OK, I hear you saying that it sounds like warmed-over Obama, and that the GOP is too far gone for reconciliation. It is also true, however, that the divisions in our country are starting to become dangerous, and that Biden is, by far, the least divisive of the first tier candidates, as he does not seek to demonize the wealthy or the mainstream of the GOP. It is further true that calls for unity and compromise resonate with a large portion of the Democratic electorate. Finally, it gives him a blueprint as to how to behave in office in the face of Republican obstruction. It preemptively makes a virtue of likely legislative failure.