The argument for a Joe Biden candidacy, as it is usually formulated, is that he is the failsafe option for Democrats concerned about the viability of Hillary Clinton. He's his own candidate, of course -- or, rather, might be, someday -- but the heavy overlap between his base and Clinton's means that a lot of Democrats will see them as either/or.

That also means that the best time for Biden to step into the Democratic field -- should he ever decide -- would be as Democrats are at their most discontented with Clinton. That would have been somewhere around Sept. 13, according to the Real Clear Politics polling average.

A worse time for Biden to join the field would be when Clinton is ascendant, when polls show that Democrats are increasingly comfortable with her as their nominee and less interested in a Biden candidacy.

In other words, a worse time is right now.

In the Post's new poll, conducted with ABC News, Clinton is seen as the clear winner of last week's debate and has regained a lot of support lost over the summer. Other recent polls echo that. And where Clinton led Biden by only 20 points in the polling average in late September, she leads him by nearly 30 now.

Since the debate, Clinton's gained almost four points, while Biden has stayed flat. But the Clinton renaissance preceded that debate; since Oct. 1, she's up over six points in the polling average. Biden is down almost three.

Again, those two things are probably linked. The second choice of Biden backers is usually Clinton, and vice versa. They're unevenly splitting a base of support. It's not entirely zero-sum; some people would bail on either in favor of Bernie Sanders. In our new poll, Clinton gains 10 points without a Biden candidacy and Sanders gains two. But the overlap (and math*) suggests that increased support for Clinton means decreased support for Biden.

If Biden really, truly plans to make an announcement over the short term, a threat we've heard often before, it's by no means too late. But it's just late enough to make it obvious that it would have been much better coming about three weeks ago.

Which, our timeline from Monday reminds us, was about when he'd originally promised a decision anyway.

* The math: The polling averages for Biden and Clinton are inversely correlated with coefficient of determination of 0.53.