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Last month, when Joe Biden’s candidacy was in tatters and speculation was swirling about whether he would stand aside, a poll suggested only one Democrat could beat Donald Trump.

That candidate, the Reuters/Ipsos poll indicated, was Michelle Obama.

The former first lady consistently polls as the most popular Democrat in America. And despite Mrs Obama repeatedly making clear she has no political ambitions, there have long been reports from within Trumpworld of concern about her fulfilling the fantasies of many Democrats and deciding to run.

And after her performance in Chicago last night, it is easy to understand why they may have been worried.

Mrs Obama lit up the second night of the Democratic National Convention on Tuesday with what some commentators declared the best political speech they had ever heard.

Speaking at the same event eight years ago, Mrs Obama famously said “when they go low, we go high”.

But last night, she and her husband took a different tack. They painted Trump as a grievance obsessed grumbler whose act had gone stale.

There was no mistake who Mrs Obama was referring to when she said that for most Americans: “If things don’t go our way, we don’t have the luxury of whining or cheating others to get further ahead… we don’t get to change the rules so we always win.”

Mrs Obama said she knew from experience that Trump is likely to resort to “ugly, misogynistic, racist lies” about Kamala Harris.

And she got a huge laugh when, referring to Trump’s comment in June that illegal immigrants are taking “black jobs”, she said: “Who’s going to tell him that the job he’s currently seeking might just be one of those black jobs?

Laughing at the former president is a new approach for Democrats, but it may be a very effective one. The change in approach is often credited to Ms Harris’s vice-presidential pick Tim Walz who has repeatedly branded Trump “weird”, a label that other Democrats have used in recent weeks.

Joe Biden often issued dark warnings that Trump poses a dangerous threat to democracy. That built up the former president as a sinister but significant figure.

Both Michelle and Barack Obama opted for the new approach by using jokes to cut him down. Their jokes were designed to cast Trump as ego-driven and petty. In their telling, he is not so much an evil menace but a self-obsessed irritation.

Like “the neighbour who keeps running his leaf blower outside your window every minute of every day”, as Barack Obama put it.

When Mr Obama, during his speech, poked fun at Trump’s comments on crowd sizes, his hand gestures made clear he was also referring to a certain part of the male anatomy – and the crowd roared with laughter.

At the same time Ms Harris was holding a campaign rally 80 miles away in Milwaukee.

In exactly the same arena in which Trump accepted his party’s presidential nomination last month. And in case anyone wanted to compare crowd sizes, the auditorium was packed out, a fact the Harris campaign pointed out to journalists.

Joe Biden declared on Monday night that November’s election was “a battle for the very soul of America”. But the Obamas offered a less ominous - and perhaps less divisive - view. Mr Obama called on Democrats to listen to the concerns of people who do not yet support Ms Harris, and warned against demonising Trump supporters.

“If a parent or grandparent occasionally says something that makes us cringe, we don’t automatically assume they’re bad people,” he said. “We recognise the world is moving fast, and that they need time and maybe a little encouragement to catch up.”

By criticising Trump but not his supporters, and by poking fun at him rather than provoking fear about him, the Obamas may have hit on a more effective way to campaign against a candidate who, just weeks ago at the Republican convention, appeared almost unassailable.