Yang's cash handout - designed to highlight his support for a "universal basic income" - is an unprecedented campaigning tactic. Also, quite possibly, an illegal one.

Yet his announcement wasn’t the most shocking moment of the night.

That came when Julian Castro, a former cabinet member in the Obama administration, went where few candidates have dared to go by questioning 76-year-old Joe Biden’s mental faculties.

"Are you forgetting what you said two minutes ago?" Castro said theatrically during an early section on healthcare.

If anyone missed the implication of his question - and few viewers would have - he repeated it several more times.

It was a nasty, unsubtle manoeuvre that simply made Castro, currently polling around 2 per cent, look desperate.

The tactic proved far less effective than when California Senator Kamala Harris attacked Biden over his record on school desegregation in the first round of debates.

And it was totally unnecessary. Biden’s weaknesses as a candidate are obvious for all to see and don’t need to be pointed out by his rivals.

The former vice-president began the debate full of energy, and performed well during the healthcare discussion.

He made a convincing case why his plan to expand Barack Obama's healthcare policy is less expensive and more likely to become law than Bernie Sanders and Elizabeth Warren's Medicare-for-all proposal.

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"She’s with Bernie, I’m with Barack,” Biden said, referring to Warren.

But as the three-hour debate dragged on so did his answers, which became increasingly circuitous and at times borderline incoherent.

Take Biden's answer on how to tackle racism: "Play the radio, make sure the television - excuse me, make sure you have the record player on at night, the - make sure that kids hear words, a kid coming from a very poor school- a very poor background will hear 4 million words fewer spoken by the time we get there," he said.

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It’s painful to read, and was painful to watch.

Biden has always been verbose and is famously gaffe-prone. But his stumbling answers feed doubts about whether he is too old to be president.

Which isn't to say that this debate will reshape the Democratic primary and send Biden tumbling from the top of the leaderboard.

He remains a flawed candidate - and the most likely Democrat to win the party's nomination.

Most Democrats still regard Biden fondly and believe he is the most likely candidate to beat Trump in the key swing states. He is in the lead despite, not because of, his debate performances.

Warren's answers were typically fluent, deftly tying together policy arguments with personal anecdotes. And very left-wing, even by progressive standards.

Several candidates - including Cory Booker, Pete Buttigieg and Beto O’Rourke - performed impressively but didn't achieve the break-out moment they needed to send their poll numbers soaring.

The fundamentals of the race appear largely static for now: Biden in front, Warren slowly rising and Sanders holding onto his hardcore base of support.

Offering his views to a local news site after the debate, Iowa Democrat Kevin Cavallin said: "Warren is a stronger candidate and I like her a lot, but the problem is, or my concern is, that she’s going to cause all of the Independents and all of the Republicans [who] are disenfranchised to go running [in] the other direction."

"Biden’s stances I like better. I think they’re more moderate, but I also think the guy’s a train wreck. The man can’t walk five feet without sticking his foot in his mouth or contradicting himself. So right now, I’m just kind of disheartened at the fact there are polar opposites on either side," he added.

It's a good summary of the problematic position Democrats find themselves in as they prepare to take on the most consistently unpopular American President since at least World War II.

This reality TV show still has many episodes to go.