If you’re Joe Biden and you’re contemplating a possible presidential run, there’s one overriding question: What’s the rationale?

What argument do you make that plausibly propels you into a contest against the long-presumed nominee? You served in the same administration, so sharp distinctions on, say, foreign policy are out. The distinctions on domestic issues are insubstantial. And, as I’ve written here before, attacks on Hillary Clinton’s “honesty and trustworthiness” simply will not fly in an intra-party battle. Republicans can and will make such arguments with abandon; Democrats can’t.

Now, however, the vice president has the rationale he needs. According to a new Wall Street Journal/NBC News poll Biden stacks up better—much better—against every probable Republican nominee than Hillary Clinton.

I know, I know: Potential candidates often poll much better when they are not actual candidates (Clinton herself was far more popular before she formally entered the campaign). And anyone relying on early polls as guides to what will happen is likely planning visits to the presidential libraries of Rick Perry, Rudy Giuliani, Herman Cain, Howard Dean, Joe Lieberman, and Gen. Wesley Clark.

What this poll does accomplish, however, is to offer Biden a case for a run that does not require him to split policy hairs or to attack frontally Clinton’s character vulnerabilities.

Reaching back to my long-ago days as a political operative, I imagine Biden’s announcement statement sounding something like this:

“I had not intended to be standing here this morning. I don’t need to remind you of the burden of loss that my family and I are carrying; and I‘ve shared with you my uncertainty about whether I had the will to conduct the kind of campaign any candidate would owe to his supporters.

“But it has become clear to me in recent days that our party—and our country—faces a threat that overrides even the most pressing personal concerns. We see a Republican Party that is at war with itself—and with the very idea of responsible governing. Its conservative Speaker of the House is now leaving, unable to deal with the most extreme elements of his own caucus. A Republican senator and presidential candidate calls his own leader a ‘liar’ on the floor of the U.S. Senate. Their party’s leading presidential contenders have somehow turned inexperience and ignorance into assets, and propose massive tax cuts for the wealthiest, and summary deportations of 11 million men, women and children.

“With control of both houses of Congress, and an effective conservative majority on the Supreme Court, this brand of politics is one election away from total control of the political process. It is a tiresome cliche to say that the next presidential election is ‘the most important of our lifetimes.’ I believe that this time, it is a simple matter of fact.

“In that circumstance, Democrats must put the strongest possible candidate at the top of the ticket. Right now, I seem to be that candidate. But I am not asking the party to rely on any polls. The way to find our strongest candidate is through a vigorous contest: the same kind of contest that gave us John Kennedy, Bill Clinton, and Barack Obama. Whoever she or he may be will be a better candidate by winning a campaign that will measure our strengths and weaknesses.”

It’s an argument that does not require Biden to raise any of the character doubts that have plagued Clinton; but it is, of course, the subtext of a call to find the strongest Democrat. It links Biden to a cause beyond himself; and it encapsulates the doubts about putting up a nominee that most Americans now find neither honest nor trustworthy.

Will Biden make that case? I have no idea. Would such a campaign succeed? I have no idea. Clinton is still favored by a plurality (if not a majority) of Democrats; the gender issue still plays out in her favor. But unlike the 2012 Clinton-Replaces-Biden-As-Veep fantasy—when there was no reasonable case Obama cold make for such a switch—there is a plausible, comprehensible argument for Biden: we have to win this election; I have the best chance of doing just that.