In remarks at the Democratic National Convention, Mrs Obama offered a sharp rebuke of the Trump presidency, telling viewers he “has had more than enough time to prove that he can do the job, but he is clearly in over his head”.
“He cannot meet this moment,” she told the convention on Monday, held online due to Covid-19.
She added that “if you think things cannot possibly get worse, trust me, they can”.
Mrs Obama emphasised the need for all Americans to vote, making reference to the voters who stayed home in 2016 and helped deliver Mr Trump victory that year, even as he lost the popular vote.
“We’ve all been suffering the consequences,” she said.
In contrast, she described Mr Trump’s Democratic presidential rival for the November election, Mr Biden, as a “profoundly decent man” who “knows what it takes to rescue an economy, beat back a pandemic and lead our country”.
Mrs Obama said a lack of empathy in leadership was having an impact on the children of America.
She said: “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.”
Former Democratic presidential candidate Bernie Sanders also unleashed a scathing attack on Mr Trump, suggesting that under him “authoritarianism has taken root in our country”.
The Vermont senator said Mr Trump had proved incapable of controlling the coronavirus outbreak, coping with its economic fall-out, addressing institutional racism in the US and tackling climate change.