“The vice president has not made a decision about his political future,” a spokeswoman for Joe Biden told the Wall Street Journal over the weekend. “Anyone speculating that he has made a decision is wrong.”

Put another way, Biden is thinking about a presidential run and thinking about it pretty seriously. And once those thoughts start running through your head, history and human nature suggest that they're hard to silence. Biden wants to be president; he has long wanted to be president, and if this is a longshot bid from a man whose previous efforts have fallen short, well, it's also the last time the 72-year-old will have any chance at all.

So a couple of questions to be addressed:

Can he win the nomination?

Biden didn't launch a presidential bid a year or six months ago because Hillary Clinton had come to seem the inevitable Democratic nominee. He's considering it now because Clinton no longer seems so inevitable. I still believe that the Clinton email story has been overblown, the consequence of a media world with little else to cover on the Democratic side. Based on everything we know to this point, a secretary of state's decision to use a private email server rather than a government email server, while a mistake, is not enough to bring down a major presidential campaign.

If that assessment proves wrong, it's because the campaign in question had other serious problems that the choice of email server merely exposed. And that may indeed be plausible.

As noted above, Biden has made short-lived runs at the presidency before, and it's hard to imagine what has changed to make the outcome different this time. He may be getting a lot of encouragement to run from inside the Washington bubble, but I don't hear any great clamor from the rest of the country. A late, under-financed candidacy along the lines being contemplated by Biden can indeed catch fire when there is a magical match between person and opportunity, but I don't see the conditions for that. By most accounts, Biden's greatest asset in Washington is an ability to generate respect and affection among those with whom he works, including those across the aisle, and while that's valuable and even admirable it's a hard attribute to sell on the campaign trail.

As vice president, Biden hasn't been associated in the public mind with any specific policy initiative.  At this point he lacks a message, a justification for his candidacy beyond personal ambition. His private meeting with Sen. Elizabeth Warren last week is an acknowledgement of where the energy lies that he must attempt to tap, the same populist energy that Sen. Bernie Sanders has excited on the campaign trail.

In some minds, Sanders is serving much the same function in 2015 as Sen. Eugene McCarthy in 1968. The surprising early success that McCarthy enjoyed in primaries against President Lyndon Johnson exposed Johnson's vulnerabilities, forced LBJ from the race and drew Sen. Robert Kennedy to announce his own candidacy. RFK was probably on his way to the nomination and perhaps the presidency when an assassin's bullet cut him down.

The problem with that model is that even if Sanders is another McCarthy and Hillary another LBJ, Joe Biden is no Robert Kennedy.

If Biden runs, does it help or hurt Democratic changes in 2016?

If Biden does run for president, it wouldn't be as the anti-Hillary. More like the non-Hillary. And as a result, I don't think the Democratic Party has anything to fear from a Biden candidacy.  At worst, he would bring new attention to the race and force the Clinton campaign to step up its game. In fact, a spirited, policy-based primary campaign among Clinton, Biden, Sanders and even Martin O'Malley would cast the party in a positive light compared to what looks to be a bloody spectacle on the GOP side.

And at best, Biden represents a failsafe for the party should Clinton continue to have problems. He would be there to step in if the top dog stumbles, ironically a role much like the one that he has played the last seven years.