First and foremost, she’s going to win the nomination because she only needs Democratic votes to win it, and Democrats still like Hillary — a lot. She looks today like a somewhat weaker general-election candidate than she did six months ago, and the Sanders surge has been fun to watch. But mostly he’s just been consolidating the party’s natural anti-Clinton bloc — white, well-educated, and quite left-wing — rather than making deep inroads into her national support.

That anti-Hillary bloc is overrepresented in states like Iowa and New Hampshire, where Sanders’ poll numbers are particularly robust, but it isn’t the basis for a winning coalition; not even close. For that, you would need a candidate capable of performing the same feat as Obama in 2008, and winning not only white liberals but a large share of minority support (an overwhelming share of the black vote, in his case) as well.

And none of Hillary’s possible rivals, real and hypothetical, are well-suited to building that kind of coalition. Sanders is already enduring left-on-left clashes with “Black Lives Matter” activists, the hapless Martin O’Malley is associated with Baltimore policing, and given the front-runner’s pre-commitment to a voting rights push and criminal justice reform, it’s not clear what Biden would offer minority voters to make them reconsider their strong support for Clinton.

Any other path to the nomination, meanwhile, requires persuading white Democratic women to turn on Hillary en masse — an even more unlikely scenario, it’s fair to say, than imagining Biden as a Rainbow Coalition candidate.

What’s more, even if the path were there, late-entering white knights have a simply terrible track record in modern presidential politics: See Fred Thompson, Rick Perry, and Wesley Clark for recent cautionary tales. And all of those men were leaping into a campaign with a much weaker front-runner, joining a more divided field. It may be possible to get into a primary race late and win; I thought someone might have done it against Mitt Romney in 2012, given how much the G.O.P. base pined for an alternative. But against a candidate as well-funded, widely-endorsed and, again, popular within her own party as Hillary? I think not.