“Donald Trump is the wrong president for our country,” said the former US first lady in an emotional recorded message to the Democratic convention.
Disaffected members of Mr Trump’s Republican party also piled in on him at the Democratic party conference.
The election takes place on Tuesday 3 November.
Because of the coronavirus outbreak, Democrats scrapped plans for a crowded party extravaganza with balloon drops and all the other political razzmatazz in Milwaukee, Wisconsin.
But it is unclear whether the largely virtual schedule of pre-recorded speeches with no live audience can generate the same level of enthusiasm as pre-pandemic gatherings of the party faithful.
Republicans will face the same challenge as they make their case for four more years in the White House at a drastically scaled-down convention next week.
Mrs Obama, who recorded her keynote address before Mr Biden announced his running mate, Senator Kamala Harris, six days ago, launched a blistering attack on Mr Trump.
“You simply cannot fake your way through this job,” she said in remarks that closed the first night of the convention on Monday.
The headline speaker added: “Our economy is in shambles because of a virus that this president downplayed for too long.”
“Stating the simple fact that a black life matters is still met with derision from the nation’s highest office,” Mrs Obama continued.
“Because whenever we look to this White House for some leadership, or consolation or any semblance of steadiness, what we get instead is chaos, division and a total and utter lack of empathy.”
She said the last four years had been difficult to explain to America’s children.
“They see our leaders labelling fellow citizens enemies of the state, while emboldening torch-bearing white supremacists.
“They watch in horror as children are torn from their families and thrown into cages and pepper spray and rubber bullets are used on peaceful protests for a photo op.”
Mrs Obama continued: “Donald Trump is the wrong president for our country. He has had more than enough time to prove that he can do the job, but he is clearly in over his head.
“He simply cannot be who we need him to be for us. It is what it is.”
She described Mr Biden as a “profoundly decent man”, touting the Democratic White House candidate’s experience as vice-president under her husband, President Barack Obama.
“We have got to vote for Joe Biden like our lives depend on it,” she said, wearing a necklace that said “Vote”.
A lot of politicians spoke at the camera during the “virtual” Democratic convention on Monday night. The only speaker who landed an emotional punch, however, was Michelle Obama.
The truth, she said, was that Donald Trump “simply cannot be who we need him to be for us”.
“It is what it is,” she said, employing the same words the president recently used about the coronavirus death toll – a jab that was as subtle as it was devastating.
She wasn’t trying to convince Republicans to switch sides. That was John Kasich’s job.
She wasn’t trying to get left-wing progressives to rally to Biden. Bernie Sanders handled that.
Mrs Obama was speaking to loyal Democrats, some of whom may stayed at home or voted for a third party in 2016, some of whom may be dispirited or scared this year.
Her goal was to drive home the gravity of the moment and to give them a call to action.
BBC
