Now we know why Barack Obama hasn't endorsed his number two for president.  Or given tacit support to the Biden campaign.  Or even acknowledged the man seeking his former position, whom he chose to lead the country in his stead in case of untimely incapacitation.

Joe Biden's dotage is showing, and not in an endearing, avuncular kind of way.  Instead of giving wet willies to tweens with a spittle-soaked, withered finger, the former vice president is liable to snog a married gal unprompted, taking the campaign idiom "pressing the flesh" literally.

Team Biden wonders, "Why won't Obama say something?"  It's because a dim realization is setting in for the 44th president: the man best positioned to carry on his legacy is suffering from a nationally broadcasted case of encephalopathy.

The past week has demonstrated that Biden is better off in Shady Acres than knocking about Iowa, trying to assume the most stressful job in the world.  The gaffes and logorrheic speeches were funny at first.  Even Biden leaned in to his nattering habit.

But his behavior over the last seven days should be eliciting pangs in every campaign aide's conscience.  Surely, the moula they're being paid isn't worth the debilitating marathon they're subjecting their boss to.

At a campaign stop in Iowa, while being introduced by his wife, Dr. Jill Biden, ol' Joe got a little too frisky and lovingly nibbled on her finger.  This behavior might be cute for, say, Chris Pratt or the late Hugh Hefner.  Not the potential leader of the free world.  Donald Trump got a lot of media obloquy for dropping the occasional yonic insult.  Now it looks as though a Biden campaign functionary mixed up Joe's Viagra and Metamucil, sticking the former in his orange juice instead of the latter.

The Trump campaign, more aggressive in rapid response than any previous Republican presidential campaigns, made hay out of the prurient peck, posting a video from 2017 of Biden attempting to tell an anecdote about confronting a gang member named "Corn Pop" as a lifeguard.

"I got hairy legs that turned ... blonde in the sun. And the kids used to come up and reach into the pool and rub my leg down so it was straight and watch the hair come back up again," Biden reminisced to a scrum of black youths.  Why he thought this factoid was relevant to his larger shaggy dog story of standing down a mythical bruiser is something explained only by the dulled synapses within his silvery head.  Biden then mentions that he "learned about kids jumping on my lap, and I loved kids jumping on my lap," at which point Child Protective Services was presumably called to safely escort the young spectators away.

This combination — the phalange-chomping and cringy, near pedophilic rambling — is enough admissible proof for the court of better judgment to rule that Biden is in no shape to be running for president.  It's hard to see how Democratic voters can justify perpetuating the campaign by keeping Biden's surname atop the polls. 

Even Biden is sensing that his limited star power is dimming.  He challenged a reporter to a wrestling match in a grasping attempt to demonstrate his vim and vigor.  Yet the headlines about his fitness continue to pop up.

Biden's biggest selling point is, much to his consternation, now in question.  Obama's veep was supposed to be running in the moderate lane, but now a new driver has emerged who checks an aggrieved minority box and can speak in short, declarative, non-discursive statements.  Worse yet, two more hopefuls just entered the race, both with Biden's unofficial strapline: I'm not a revolutionary.  One might be even closer to Obama than the V.P. himself. 

Where does that leave Uncle Joe, besides seated by himself mumbling at Christmas dinner?  The Democratic heart has its hopes pinned to senators Warren and Sanders.  The Democratic head looks to Pete Buttigieg or Senator Amy Klobuchar as acceptable choices who can win back the Midwest.

Biden is somewhere between, a nostalgia candidate who recalls the calmer years when Democrats controlled the White House and the media didn't pull the national fire alarm every day.  But this too is a liability, especially for a party increasingly uncomfortable with Washington's chummy golden years, when segregationist Democrats and Northeastern Republicans reached concords in smoky rooms.

So what now?  Does Biden finally pack it in and, at last, enjoy his retirement from public life?  Hardly.  He's still the frontrunner of the field.  One of his non-socialist rivals just ended her campaign. 

Chances are still high that Democrats draft him for the general election.  But I can't imagine many voters are chuffed by the prospect of Biden helplessly verbigerating on the debate stage, a more than easy target for the unforgiving Trump.