Can a candidate who is leading the field by 12 points be weak? That's the question of Joe Biden's run for the Democratic presidential nomination.

The fact is, Biden's campaign is showing signs of weakness. So is the candidate himself. And doing pushups, as Biden suggested he might do to show up President Trump, won't make him any stronger.

In mid-May, Biden led the sprawling Democratic field by 26 points in the RealClearPolitics average of national polls. By late May, the lead was 17 points. By late June, it was 15 points. Today, it is 12.

So Biden's lead is still big, but it's less than half what it was not that long ago.

What might be more concerning to Biden supporters are the signs Biden is not running at full strength, that the 76-year-old former vice president has lost a step.

Some voters became concerned about that at the first Democratic debate on June 27, when rival Sen. Kamala Harris hit Biden with an attack about his opposition to busing to desegregate schools in the 1970s. (Harris began the exchange with a zinger: "I do not believe you are a racist.") Even though Biden's old position had been in the news, he seemed unprepared. His response was feeble and disorganized, ending with, "My time is up. I'm sorry."

Afterward, Biden complained that there was no way to explain a nuanced issue in the 30 seconds required by a televised debate.

In a conversation Tuesday with MSNBC's Mika Brzezinski, Biden admitted he was blindsided by Harris. "I didn't expect that particular attack," Biden said, "and that question starting off, 'I know you're not a racist.'" He complained again about debate time limits.

And then, Brzezinski asked, "The public is interested in how you're going to fight Donald Trump. Kamala Harris — if you weren't ready for her, are you ready to fight this guy who doesn't play fair?"

"My strategy is just to go out," Biden responded, "and remember that famous expression my grandpop used to use, when they said to Harry Truman, give 'em hell, Harry, and he said no, I'm just going to tell the truth, they're going to think it's hell."

Biden explained that he expects the campaign to be ugly and that Trump will not run an "honorable" race. "But I'm not backing down at all from him," Biden said. Yes, he was surprised when Harris "said something so off the wall to me," but Biden pledged to "take on" the president.

It wasn't exactly confidence-inspiring. And then, Brzezinski asked what Biden would do if Trump "starts making fun of your age, your mental state, starts going after you ..."

"I'd say, 'Come on, Donald,'" Biden said. "'Come on, man. How many pushups you want to do here, pal? I mean, jokingly. Run with me, man.'" Biden went on to explain that he often runs when he takes part in parades so he can make sure to meet as many people and shake as many hands as possible.

It was the answer of a man who knows the public is concerned about his vigor. Yes, Trump himself is pushing the limits of presidential age — he was 70 when he took office. But if Biden is elected, he will be 78 upon starting the presidency — the same age Trump would be upon finishing a second term.

At a recent Harris town hall in South Carolina, a number of voters — not just Harris supporters but also Democrats still shopping for a candidate — expressed doubts about Biden's staying power.

"He's getting slow," said one voter. "He can't think that quick, on the go. He gets shaken. I don't think he could beat Trump on stage in a real debate. I think Trump would just tear him up."

"I think he's dropping off," said another. "He's losing momentum right now."

"We think maybe his time has come and gone," said yet another, speaking for himself and his wife.

No number of pushups will fix that.

That is not to say that Biden is without strengths. He is still well ahead in the national polls, as well as early-state polls in Iowa, New Hampshire, and South Carolina. He is still raising a lot of money, although much of it comes from large donors who have hit the legal contribution limit, while other candidates have lots of small donors who can give again and again and still stay under the limit.

But Biden, the man who has spent more than four decades in federal office, still has to convince voters he has what it takes for four, and perhaps eight, more years. So far, he hasn't gotten that job done.

