“Donald Trump is the wrong president for our country,” she said.
“He cannot meet this moment.
“He simply cannot be who we need him to be for us.”
The former first lady, one of the nation’s most respected women, was the headliner at the first presidential nominating convention of the coronavirus era.
There was no central meeting place or cheering throng during the all-virtual affair.
But it was an opportunity for Democrats, and some Republicans, to rally behind Mr Biden, the party’s presidential nominee.
Bernie Sanders, the progressive Vermont senator who was Mr Biden’s last standing rival during the primary, encouraged his loyal supporters to vote for the former vice president in November, arguing the nation cannot survive another four years of Mr Trump.
He notably backed Mr Biden’s plan for tackling health care, one of their most substantive differences in the past.
Mr Sanders backs a Medicare for All plan while Mr Biden has called for expanding the current “Obamacare” law.
But it was Michelle Obama, making her fourth convention appearance, who once again delivered an electrifying moment.
Wearing a necklace that said “vote”, she tapped into her enduring popularity among black voters and college educated suburban women, voters Mr Biden will need to show up in force.
She issued a stark warning to a country already navigating health and economic crises along with a reckoning on racism.
“If you think things possibly can’t get worse, trust me, they can and they will if we don’t make a change in this election,” she said as she issued a call to action for the coalition of young and diverse voters who twice sent her family to the White House.
Mr Biden will formally accept the nomination on Thursday near his home in Wilmington, Delaware.
His running mate, California Senator Kamala Harris, who is the first black woman on a national ticket, speaks on Wednesday night.
We will not let Trump sabotage our Postal Service or destroy our democracy.
— Bernie Sanders (@SenSanders) August 13, 2020
Leading up to that, Mr Biden sought on opening night to demonstrate the broad ideological range of his supporters.
On the same night he was praised by Mr Sanders, a self-described democratic socialist who championed a multi-trillion-dollar universal health care plan, Mr Biden also won backing from Ohio’s former Republican governor John Kasich, an anti-abortion conservative who spent decades fighting to cut government spending.
“My friends, I say to you, and to everyone who supported other candidates in this primary and to those who may have voted for Donald Trump in the last election: The future of our democracy is at stake.
“The future of our economy is at stake. The future of our planet is at stake,” Mr Sanders said as he endorsed Mr Biden’s health care plan.
Still, there were real questions about whether the prime-time event would adequately energise the disparate factions Mr Biden hopes to capture.
Republicans face a similar challenge next week.
Mr Trump sought to undermine the Democrats’ big night by hosting a political rally in Wisconsin, where Mr Biden’s party had originally planned this week’s convention.
He called the Democrats’ event “a snooze” before it even began.
Monday’s speeches were framed by emotional appearances from average Americans touched by the crises that have exploded on Mr Trump’s watch.
Philonise and Rodney Floyd led a moment of silence in honour of their brother, George Floyd, the Minnesota man whose death while in police custody sparked a national moment of awakening on racial injustice.
“George should be alive today,” Philonise Floyd said matter-of-factly.
Also speaking was Kristin Urquiza, an Arizona woman who lost her father to Covid-19, which has killed more than 170,000 Americans as of Monday evening.
“My dad was a healthy 65-year-old,” she said.
“His only preexisting condition was trusting Donald Trump, and for that, he paid with his life.”
Mrs Obama, whom Gallup determined was the nation’s most admired woman last year, wowed Democrats at the 2016 presidential convention by coining the phrase: “When they go low, we go high”.
She insisted on Monday she was not abandoning that tack, explaining that taking the high road does not mean staying silent.
With no live audience for any of the speakers, convention organisers were forced to get creative in their high-stakes quest to generate enthusiasm.
There were live appearances from speakers in Texas, South Carolina, Wisconsin and Michigan, but many of the speeches that aired Monday night were prerecorded.
Seeking to inject some family fun into an otherwise serious two-hour video montage, the campaign hosted drive-in viewing stations in six states, much like drive-in movies, where viewers could watch on a big screen from the safety of their vehicles.
There were also many online watch parties featuring celebrities and elected officials to make the experience more interactive.
Monday’s speakers included plenty of Democratic politicians: representative Jim Clyburn of South Carolina, who is the highest ranking African American in Congress; New York governor Andrew Cuomo; Michigan governor Gretchen Whitmer; Alabama senator Doug Jones; Nevada senator Catherine Cortez Masto and two former presidential contenders: Minnesota senator Amy Klobuchar and Mr Sanders.
And beyond Mr Kasich, there were three high-profile Republicans backing Biden who got speaking slots: California businesswoman Meg Whitman, former New Jersey governor Christine Whitman and former New York congresswoman Susan Molinari.
