(Leigh Vogel/Getty Images)

What will Joe do?

The Joe in question is, of course, Joseph Robinette Biden Jr., the vice president of the United States. And the “do” refers to whether he will take the plunge and enter the 2016 presidential race. If he does, just how much would he endanger Hillary Rodham Clinton’s faltering prospects for the Democratic nomination?

Biden had initially set the end of the summer as a decision date but has repeatedly pushed back that timeline. Despite CNN making clear that he would be welcome at the first Democratic presidential debate Oct. 13, the vice president’s political team said he will skip it even if he decides to run. (Biden isn’t interested in operating on a TV network’s timetable for the biggest decision of his life; “he won’t be pushed,” a longtime Biden adviser said.)

Biden has made clear that he is grappling with what to do — a particularly difficult decision, given that his eldest son, Beau, passed away in May and said before his death that his dream was to see his father run for president one more time. (Biden ran briefly in 1987 and again in 2008.)

Even putting aside the deep emotional conflict playing out for Biden, the question of whether he could win is a tough one to answer — and may be responsible for the delay in his decision.

Let’s go through what we know about the state of the race first:

1. Clinton has shown surprising and significant weakness since she entered the race in April. Clinton, who many expected to coast largely unopposed to the nomination, is behind Sen. Bernie Sanders (I-Vt.) in New Hampshire primary polling and is struggling to put distance between herself and her onetime Senate colleague in Iowa.

Much of Clinton’s weakness is based in her inability to put questions behind her about the propriety of a private e-mail server she used while she was secretary of state. With Clinton set to testify on Capitol Hill this month, it’s hard to see the e-mail problems going away anytime soon.

2. Sanders, whom many dismissed as a political gadfly at the start of the race, has tapped into a desire among liberals to have a straight-talking champion. He drew 20,000 people in Boston on Saturday — one of many large crowds that have gathered nationwide to see him speak.

Most surprising — at least to me — is that Sanders raised $26 million over the past three months despite holding only seven official fundraisers. (Clinton raised $28 million from 58 fundraisers.)

3. Biden would enter the race in third place almost everywhere. In new NBC-Wall Street Journal polls in New Hampshire and Iowa released Sunday, Biden is at 18 percent in the former (10 points behind Clinton for second place) and 22 percent in the latter (six points behind Sanders).

What those polls also show is that if Biden joined the race, Clinton’s path to the nomination would be tougher. Clinton leads Sanders by 11 points without Biden in the race and by only five with him in it in Iowa; in New Hampshire, Clinton trails Sanders by nine points in a Biden-less race and by 14 points with the vice president running.

Viewed broadly, what’s clear is that if Biden ran, he would be virtually guaranteed to play a spoiler role. He would take away more votes from Clinton than from Sanders — lowering the win margin for the senator and, conversely, moving the win number further away from Clinton.

But does the second-most powerful politician in the country — especially one who spent decades in the Senate before moving into national office — really want to be nothing more than a spoiler in his final race?

Although the Biden people who know what he’s thinking aren’t talking, it’s hard to imagine that someone who is as proud as Biden and as committed to the Democratic Party would enter the race knowing that the best he could do would be to keep Clinton from the nomination.

Whether Biden could actually win is the key question — and also the one that is impossible to answer with him sitting on the sidelines.

What we know is that the idea of a Biden candidacy wins the backing of about one in five Democratic primary voters nationally and in key early-voting states.

What we don’t is how a Biden candidacy would affect those numbers. It seems likely that Biden would benefit from an initial boost in his numbers because of the attention that announcement would garner.

But what happens from there? Are undecided Democratic voters reminded of the things — his tendency to just say stuff, his long years spent in official Washington, etc. — that made Biden less than appealing as a candidate for the big job during his past two bids?

Or would a Biden candidacy lead to a reevaluation of Clinton by many establishment types who have stayed with her simply because they can’t imagine backing Sanders? Could Biden, under this scenario, trigger a full-scale revolt against Clinton by some (or many) of the establishment building blocks of her candidacy?

Without Biden in the race, it’s impossible to know which of those scenarios — or a third or fourth possibility — is most likely. That reality may well be the biggest reason Biden keeps delaying his decision — searching for some signal of how the race might play out if he were in it.

Fortune tends to favor the bold in politics. But is Biden willing to take the biggest risk of his life at age 72?