WASHINGTON - In one minute, Joe Biden seems like a presidential candidate-in-waiting, eating up adoration from die-hard supporters who are pleading with him to run. The next minute, he seems light-years away from convincing himself he's ready to run - a man still reeling from personal tragedy.

In painfully public fashion, Biden is wrestling with whether to close the curtain on a political career that's spanned nearly half a century, or to fight one last fight for the job he's always wanted. In a stroke of harsh irony, he's at the height of his political popularity at the moment he's least prepared to capitalize on it.

Confounding Biden's decision about 2016 are emotional aftershocks from his son's recent death, which Biden has begun to discuss publicly in a stunning instance of a politician unmasking his own frailty. This week he revealed that during a recent trip to Colorado, he "lost it" when a well-wisher brought up Beau Biden's decorated military service in Iraq.

"You can't do that," Biden told talk-show host Stephen Colbert on "The Late Show."

He was alluding to an unforgiving reality of American politics: Most voters want their presidential candidates to exude strength and self-certainty - not some of the time, but all of the time.

Whether Biden, over the next weeks and months, can muster the emotional strength needed to campaign for president at full speed remains the biggest question in his deliberations, several of his advisers say. Part and parcel to that consideration is the toll a campaign could take on his family, say the advisers, who requested anonymity because they weren't authorized to discuss the topic publicly.

Biden has made no secret of the fact that barely three months after his son died from brain cancer, waves of grief still wash over him at moments not always predictable. He now peppers most of his speeches with references to Beau's impressive life and career, sometimes seeming on the verge of tears. On the anniversary of the Sept. 11 terrorist attacks on Friday, he expressed deep gratitude to firefighters in Manhattan who offered him consolation over his son's death.

"I know it's a tough day for you," Biden told the unit, which lost nearly half its firefighters on 9/11. "Believe me, I get it."

In baring his soul so publicly, Biden has set a high bar for himself to surmount should he choose to get in the race. Having established a litmus test for presidential candidates - they must be "willing to give it 110 percent of who they are" - he would have to explain what's changed after saying this week that he'd "be lying if I said that I knew I was there."

Jim Manley, a veteran Democratic strategist, said Biden's interview with Colbert was an outlier in an environment in which most politicians avoid situations where their emotions will be on display. Still, Manley said, he would discourage Biden at this point from getting in the race.

"It was obvious from the interview that he is very conflicted, which is a dangerous place to be for someone who wants to run for president in the age of 24/7 scrutiny," Manley said.

As he ruminates, Biden is being cheered on by growing numbers of supporters at every turn, in what amounts to his political swansong should he ultimately decide against running.

His face has been plastered on newspaper front pages and cable news screens for weeks as his poll numbers climb - all without even being a candidate. Visiting New York this week for speeches, 9/11 commemorations and a fundraiser, he was greeted at nearly every event by chants of "Run, Biden, Run." Supporters of Draft Biden, the super PAC working to lure him into the race, stood outside the Ed Sullivan Theater with signs bearing his name as he arrived for "The Late Show."

Yet others who have been loyal to Biden have said that while they would support him unquestioningly if he runs, they're hoping he will not. One longtime Biden donor said he and others were discouraging Biden from running out of concern over what it would do to him if he lost. The person requested anonymity to discuss private conversations.

As Hillary Rodham Clinton's campaign has struggled over the summer, a growing number of influential Democrats have disclosed that they'd like Biden to challenge her - either as an alternative or to force her to step up her game. Clinton's campaign, meanwhile, has pressed donors and delegates to pledge their loyalty to her.

In publicly playing down the likelihood he'll run, Biden has sent a message of caution to Democrats that they'd be going out on a limb by backing a campaign he may not be ready to wage.