US presidential debate: Donald Trump is a sinking ship, and he wants to take everyone else down with him

In 2012, during the second presidential debate between Barack Obama and Mitt Romney, the two men bickered testily about whether or not Mr Obama had used a specific turn of phrase. It was talked of as a turning point.

This year, during the second presidential debate between Donald Trump and Hillary Clinton, one candidate threatened to throw the other in jail, and then accused the moderators of being in league with his opponent.

Trump’s task was to claw back some respectability after the most disastrous fortnight for any presidential campaign in modern times, and Republicans may have found him sufficiently plausible in the last hour to decide against trying to drop him from the ticket.

But that first 20 minutes was biblical. When he wasn’t lurking angrily like a man waiting in line for the only loo at Nando’s, Trump was spitting venom at Clinton, referring to her husband's alleged victims in the studio audience, or repeating his mantra regarding the Tic Tac tape: “locker room talk, locker room talk, locker room talk…”

If he thought he could flap the unflappable former Secretary of State, a woman who has spent several decades weathering worse, then he was mistaken. She lobbed a few accusations of her own, tying the Trump campaign to Russian hackers and to a rise in racist bullying.

All the stroppy talk of Clinton’s emails and Bill's purported historical abuses may have fed the rage of Trump’s base, but it’s unlikely to have won over any soccer moms disgusted by his comments about women, or veterans troubled by his attacks on the Khan family, or low-income workers turned off by his tax returns.

In what might have been one more first for a televised presidential debate, the property developer even threw his own running mate under the bus, suggesting he had never discussed the Syrian crisis or US-Russian relations with Mike Pence, and that he disagreed with him on both issues.

Clinton, when not parrying her rival’s insults, delivered some semi-specifics about Obamacare, the tax code, fighting Isis, tackling Islamophobia, energy and the Supreme Court. But the policy differences are almost beside the point. As this spectacle so starkly demonstrated, voters have a choice between a presidential candidate and a malevolent child.

Trump is going down, and he intends to take everyone else with him: Hillary and Bill, Mike Pence, the Republican party, the dignity of the presidency, the political culture of the United States. If America is a shining city on a hill, then he cast a long shadow across it last night.