Joe Biden’s mental acuity will be the elephant in the debate room Sunday night, after the former vice president fumbled his way through his first online town hall.

“Am I live?” the former vice president asked Friday night, as an Illinois voter tried to ask him a question.

“Am I on camera?” he asked later — shortly before wandering out of range and off viewers’ screens.

Supporters on both sides are bracing for potential fireworks in the first one-on-one debate of the Democrats’ grueling 2020 nomination race as Sen. Bernie Sanders scrambles to regain his former front-runner status.

“I don’t think this thing should happen,” veteran Democratic strategist James Carville told The Post. “If you really go after Biden and call him corrupt and war monger and anything you say, [Sanders] might have a 1.2% chance to win but that’s it.”

Sanders will hit Biden on policy, but won’t slam him on his increasingly frequent campaign-trail gaffes, said progressive activist Jordan Uhl.

“A lot of people on the left want him to do it, go scorched earth,” he said — but that tactic could backfire.

“A lot of older voters would get defensive and recoil,” he said.

The former vice president is crushing Sanders in the latest polls out of the four states set to hold Democratic primaries on Tuesday.

The underdog’s best hope may be in Arizona, where he has strong support among Hispanic voters — but even there, Biden is beating him by 17 points, last week’s Univision/ASU poll found.

And delegate-rich Florida could be a Biden romp, with five recent surveys giving him leads of 40 percent or better among the state’s moderate Democratic base — which was infuriated by Sanders’s recent defense of Fidel Castro’s communist Cuba.

Biden leads in the delegate count 890 to 736 after winning four out of six state contests last Tuesday and fighting Sanders to a draw in Washington state. He needs 1,991 delegates to win the nomination at July’s Democratic National Convention in July.

But Sanders shows no signs of throwing in the towel.

“You don’t drop out in the sixth inning just because you’re down,” Uhl said. “He’s got the resources, money and donor base. Just keep fighting, that’s it.”

The party’s rules for caucuses and primaries give Sanders an incentive to stay in the hunt: even a losing effort in each of the remaining 30 contests will add delegates to his total haul, as long as he can claim at least 15 percent of the vote — a hurdle he should clear easily, even in tough territory like Florida.

With a solid chunk of convention delegates at his back, “he’s got leverage and he can help shift the Democratic platform left,” Uhl said.

There’s also the chance that the 77-year-old Biden could be felled by a serious health problem or ruined by a crippling gaffe in the four months before the convention formally selects the party’s nominee.

“Joe Biden is not the best orator, so you never know,” Uhl said. “I don’t wish any ill-will toward him medically, but who knows what could happen.”

Here is what the latest polls show for Tuesday’s primaries: