Barack Obama used his Democratic convention speech to brand President Donald Trump as someone who never took his office seriously, putting US democracy in danger.
Mr Obama said that on handing over the White House to Trump in 2017, he thought the Republican "might show some interest in taking the job seriously; that he might come to feel the weight of the office and discover some reverence for the democracy that had been placed in his care.
"But he never did," the former president said.
Mr Obama accused Mr Trump of leaving the US's "worst impulses unleashed, our proud reputation around the world badly diminished, and our democratic institutions threatened like never before".
President Trump angrily retaliated at the accusations from his predecessor, telling reporters that Mr Obama was an "ineffective" and "terrible" leader.
"I see the horror that he has left us, the stupidity of the transactions that he made," Mr Trump said. "President Obama did not do a good job.
"The reason I am here is because of President Obama and Joe Biden," he told a press briefing.
Former first lady Hillary Clinton, who lost the 2016 presidential election to Mr Trump, will also address the convention, which has been forced due to coronavirus safety measures to be staged almost entirely online.
Others on the bill include Massachusetts Senator Elizabeth Warren, who unsuccessfully challenged Mr Biden for the nomination, and Nancy Pelosi, speaker of the House of Representatives.
The former US president will be speaking two days after his wife, Michelle Obama, opened the convention with a scathing takedown of Mr Trump, painting him as a man who lacks the competence, character and decency for the job.
"Donald Trump is the wrong president for our country," she said. "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."
