When one candidate enjoys an enormous lead in the polls in a field of two dozen, he's bound to be a target on the debate stage — the one person everyone has an incentive to take down a peg. And so it was that in Miami on Thursday night, in the second half of the first Democratic debate, former vice president Joe Biden became a punching bag.

It started with Rep. Eric Swalwell taking a dig at the 76-year-old Biden's age by calling on him to "pass the torch" to a younger generation. Then Sen. Michael Bennet hit Biden for bragging about his role in striking a budget deal with Mitch McConnell in 2011. Bennet called it a "complete victory for the Tea Party," a "great deal" for McConnell, and the key to Republicans permanently extending tax cuts passed during the administration of George W. Bush.

But most stunning of all was the extended clash between Biden and Sen. Kamala Harris over Biden's recent remarks harking back to the good old days of senatorial deal-making with segregationists — an exchange that quickly evolved, at Harris' direction, into an argument about Biden's opposition to busing during the 1970s. Biden's defense was reasonable — he had no objection to states and localities choosing to integrate schools by busing students between urban and suburban school districts but merely opposed federally mandated busing by the Department of Education. It didn't matter. Harris came off as clear, passionate, and personally invested in the issue. (A child of a Tamil Indian mother and Jamaican father, Harris was bused to school as a child in Berkeley, California) Biden looked and sounded defensive, back on the ropes.

It's how he looked and sounded all night long.

On a debate stage filled with candidates who sounded like they favored open borders, wanted to eliminate private health insurance, and intended to enact laundry lists of expensive new government programs, an alternative who staked out somewhat more moderate positions while also deftly reminding the audience of his role serving for eight years as second-in-command to a hugely popular recent president should have done well. It's no doubt why he's been doing so well up to now.

But it's hard to see how that continues. Biden has always seemed a little shaky, a little lacking in focus and discipline. He has a reputation as a one-man gaffe machine because he often speaks before he thinks. He's a politician of impulse and instinct, and his impulses and instincts have frequently been just a bit ... off. But now, entering his late 70s, gearing up for what could be a 12-month battle with a range of talented Democrats, many of whom are sharper and more nimble than he is and who are perfectly at ease on the far side of the "woke" divide — now Biden is just plain out of his depth. Or so it appeared on Thursday night.

His answers to questions posed by the debate moderators were generic Democratic-candidate word salads sprinkled with biographical boilerplate and often nonsensical references to the diabolical Trump and the saintly Obama. And that's when he was free to say anything he wanted, without an opponent pushing back and cornering him. When the latter happened, it produced an ugly, self-protective mess of verbiage.

Harris came off very well Thursday night. So did South Bend Mayor Pete Buttigieg. Even Sen. Kirsten Gillibrand sounded competent in a blandly comforting way. But not the man who, going into the debate, led the field by a wide margin. That has to make a difference to his standing in the polls.

After all the Biden-bashing in Miami, the former senator and vice president doesn't look quite so "electable" anymore.