As the buzz about Vice President Joe Biden mulling a presidential race takes flight, those sympathetic to his run float a one-term trial balloon. It’s smart on many levels and solves several problems both for Biden and the Democratic Party, the latter which is reeling from the FBI inquiry into Hillary Clinton’s e-mail and her declining poll numbers.

The latest Fox News poll shows Hillary Clinton dropping to 49 percent, with Sen. Bernie Sanders (I-Vt.) up to 30 percent. The 19-point gap between the two is down from 46 percent at the end of June. More worrisome, in general election match-ups Clinton now trails Jeb Bush (44 to 42 percent) and Sen. Marco Rubio (46 to 44 percent). Some 58 percent believe she lied about her e-mails, only 2 percent think she told the truth and 33 percent think there is some other explanation. By a 54 to 37 percent margin, respondents think she jeopardized national security.

In short, it is increasingly possible she will fall beyond Sanders nationally. If she limps through the primary, she would be vulnerable to multiple GOP contenders. Put differently, Democrats no longer have the luxury of denial. They need to have a backup plan. Hence, the “Biden as a one-termer” plan emerges.

The most obvious purpose of a one-term plan is as a preemptive response to the age issue. The Post observed, “Biden is 72 years old and would be 78 upon completing his first term. That’s five years older than the oldest president to be sworn in for a second term — Ronald Reagan, at 73. Which, we’ll note, is how old Hillary Clinton would be at her second inauguration.” But then age seems not such a big concern for Democrats. (Sanders is 73.)

More important, however, is the appeal to the cadre of not-yet-ready presidential candidates who would rather not wait eight years to take their shot. With Clinton or a GOP winner, Sens. Elizabeth Warren (D-Mass.), Kirsten Gillibrand (D-N.Y.) and Amy Klobuchar (D-Minn.) and other aspirants get the benefit of a few years of preparation and maybe a shot at the VP spot. They therefore have less reason to oppose a Biden run, even if they are nominal Clinton supporters.

And finally, Biden positions himself as the quintessential pinch hitter. If Clinton goes down swinging due to scandal or a plain-old lack of likability, Biden is there — an experienced hand with national name recognition who is simply doing the party a favor.

As stale and lacking in political skills as Clinton may be, we would not be here if not for the e-mail debacle. Rep. Trey Gowdy (R-S.C.), appearing on “Fox News Sunday,” put responsibility directly with her “for having her own personal server, for exclusively using private email, for telling us the Sidney Blumenthal emails were unsolicited and then we later find out that they were not. For telling us there was no classified information and we later find out that there was. For telling us the public record was complete and then we find 15 emails she never turned over to the State Department.” As he put it, “Sometimes when people are frustrated they look in the mirror and engage in a little bit of self-reflection and then sometimes they lash out and blame nonexistent right-wing conspiracies, and apparently she’s chosen to do the latter.”

No matter who one believes is responsible for her current travails, Clinton is a seriously damaged candidate. We wouldn’t have the prospect of a gaffe-plagued, aging VP running to become an automatic lame duck if she weren’t. And given the choice between scandal-scarred Clinton and a loyal VP, Democrats would be foolish to reject the hand Biden seems prepared to offer his party.