Democratic presidential candidate Joe Biden and running mate Kamala Harris have attacked "whining" President Donald Trump as an incompetent leader who has left the US "in tatters".
The pair held their first campaign event together, a day after Mr Biden unveiled Ms Harris as his number two.
President Trump hit back, saying Ms Harris had "dropped like a rock" in her own presidential bid.
Mr Biden will face Mr Trump, a Republican, in November's election.
What did Biden say?
Wednesday's event at a school in Wilmington, Delaware, was not open to the public, with Mr Biden, 77, citing coronavirus prevention needs. Both candidates walked on stage wearing masks to address a group of masked journalists.
Mr Biden noted that Ms Harris, a US senator from California, was the first woman of colour to serve as a presidential running mate for a major US party.
Mr Biden said: "The choice we make this November is going to decide the future of America for a very, very long time."
He continued: "Donald Trump has already started his attacks, calling Kamala, quote, nasty, whining about how she is, quote, mean to his appointees.
"It's not a surprise because whining is what Donald Trump does best, better than any president in American history.
"Is anybody surprised that Donald Trump has a problem with a strong woman, or strong women across the board?"
He also attacked Mr Trump's handling of the coronavirus pandemic, climate change, the unemployment rate and "his politics of racist rhetoric that appeals to division".
What did Harris say?
Coming to the podium next, Ms Harris said: "I am ready to get to work."
The 55-year-old former prosecutor told reporters: "Everything we care about, our economy, our health, our children, the kind of country we live in, it's all on the line."
Ms Harris - the daughter of immigrants from India and Jamaica - continued: "America is crying out for leadership, yet we have a president who cares more about himself than the people who elected him.
"He inherited the longest economic expansion in history from Barack Obama and Joe Biden. And then, like everything else he inherited, he ran it straight into the ground."
A bare-knuckle fight
Much has been made of the differences between Joe Biden and Kamala Harris since he announced her as his running mate on Tuesday afternoon. Biden is more than 20 years her senior. He's a white man, son of working-class parents in Pennsylvania. She's a multiracial woman from California, daughter of immigrants from Jamaica and India.
In their first joint appearance on Wednesday, however, they both spoke about what they had in common. "Her story is America's story," Mr Biden said. "Different from mine in many particulars, but also not so different in the essentials."
Ms Harris echoed this, saying that the two candidates are "cut from the same cloth". It's the kind of message you'd imagine would come from a campaign that's calling for national healing and unity.
Of course, that didn't stop either candidate from going on the attack against their general election opponents. It's the first salvo for this ticket in what is sure to be a bare-knuckle fight for the White House.
How did Trump respond?
At a White House news conference, Mr Trump disparaged Ms Harris' failed bid for the Democratic party's presidential nomination, during which she fiercely attacked Mr Biden's record on race.
Mr Trump said: "I watched her poll numbers go boom, boom, boom, down to almost nothing, and she left angry, she left mad.
"She said horrible things about him, including accusations made about him by a woman, where she, I guess, believed the woman.
"Now all of a sudden she's running to be vice-president saying how wonderful he is."
Last year Mr Biden was accused by a number of women of unwelcome contact, including touching and kissing. Mr Biden acknowledged at the time that he must respect personal space.
A more serious allegation surfaced this year from a former aide who accused Mr Biden of sexually assaulting her in the halls of Congress in 1993. He denied the claim.
Ms Harris, who had dropped out of the presidential race at that point, said Tara Reade "has a right to tell her story".
California state records also show Mr Trump donated to Ms Harris when she was a candidate for the state's attorney general in 2011 and 2013, while he was still a private citizen.
Shortly before the Biden-Harris event, Mr Trump goaded Mr Biden for remaining at home for much of the campaign so far amid the pandemic lockdown.
During a White House meeting with teachers who said that children learnt better in class than remotely, the president asked a asked: "So if you're a presidential candidate and you're sitting in a basement and you're looking at a computer, that's not a good thing?"
He also tweeted an attack on the Biden-Harris ticket, saying they would put Senator Cory Booker, who is black, in charge of low-income housing. Critics said the tweet was racist.
What happens next?
Mr Biden will formally accept the Democratic presidential nomination at next week's convention, which will largely be a virtual event because of the pandemic.
He became the party's presumptive nominee in April after Bernie Sanders, his only remaining competitor, ended his own campaign.
Meanwhile Mr Trump will be nominated for a second four-year term in the White House by his fellow Republicans at their party convention a week later.
A 10-week campaign will follow before voters deliver their verdict in the general election on 3 November.
Mr Trump and Mr Biden will hold three debates in September and October. Ms Harris will debate Vice-President Mike Pence in October.