Barack Obama accused his successor of using the presidency to help no one “but himself and his friends” in a recorded message to the Democratic Convention.
The third day of the conference also saw Kamala Harris officially accept the nomination as the vice-presidential candidate.
But it is the strength of the attack by former president Obama on Donald Trump that will cause the biggest shock waves of the Convention so far.
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Traditionally ex-presidents have been reluctant to criticise the current incumbent of the White House, but Mr Obama accused Mr Trump of having “no interest in treating the presidency as anything but one more reality show that he can use to get the attention he craves”.
He said: “I have sat in the Oval Office with both of the men who are running for president. I never expected that my successor would embrace my vision or continue my policies.
“I did hope, for the sake of our country, that Donald Trump 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. He’s shown no interest in putting in the work; no interest in finding common ground; no interest in using the awesome power of his office to help anyone but himself and his friends; no interest in treating the presidency as anything but one more reality show that he can use to get the attention he craves.
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“Donald Trump hasn’t grown into the job because he can’t. And the consequences of that failure are severe. 170,000 Americans dead. Millions of jobs gone.
“Our worst impulses unleashed, our proud reputation around the world badly diminished, and our democratic institutions threatened like never before.”
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It prompted an angry response from Mr Trump on Twitter during Mr Obama’s message.
Writing two tweets in capital letters, the US president claimed Mr Obama “spied on my campaign…and got caught”.
He also wrote: “Why did he refuse to endorse Slow Joe until it was all over, and even then was very late? Why did he try to get him not to run?”
Earlier at a White House news conference, Mr Trump described Mr Obama’s presidency as “ineffective” and declaring that the reason he was president was because Mr Obama and Joe Biden “did such a bad job”.
“I see the horror that he’s left us and the stupidity of the transactions he made,” he said.
“Look at how bad he was, how ineffective a president he was, he was so ineffective, so terrible.
“Now President Obama did not do a good job, and the reason I’m here is because of President Obama and Joe Biden.”
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Mr Obama’s comments have threatened to overshadow what was supposed to be the main event of the Democratic Convention today, the confirmation of Ms Harris as the vice-presidential candidate.
She told the convention of the vision she shares with Mr Biden for the future of the US.
“A vision of our nation as a beloved community – where all are welcome, no matter what we look like, where we come from, or who we love.
“A country where we may not agree on every detail, but we are united by the fundamental belief that every human being is of infinite worth, deserving of compassion, dignity and respect.”
And she of the “awesome” responsibility and privilege of sharing the ticket with Joe Biden.
“So, let’s fight with conviction,” she said. “Let’s fight with hope. Let’s fight with confidence in ourselves, and a commitment to each other. To the America we know is possible. The America, we love.
“Years from now, this moment will have passed. And our children and our grandchildren will look in our eyes and ask us: Where were you when the stakes were so high?
“They will ask us, what was it like?
“And we will tell them. We will tell them, not just how we felt.
“We will tell them what we did.”
Analysis: With excoriating critique of Trump, Obama went further than ever before
By Cordelia Lynch, US correspondent
It was an historic moment for America and Kamala Harris – the first black woman on a major party’s presidential ticket.
The room in Delaware where she delivered her barrier-breaking speech was almost empty. But Harris tried to fill it with personal stories, introducing herself to a party and a nation that is still getting to know her.
“I keep thinking about that 25-year-old Indian woman … who gave birth to me at Kaiser Hospital in Oakland, California,” she said. “On that day, she probably could have never imagined that I would be standing before you now speaking these words: I accept your nomination for Vice President.”
The montage that preceded her was littered with messages from her family and she in turn praised them. There was even a shout-out to her college sorority Alpha Kappa Alpha, the nation’s oldest African American Greek-lettered sorority.
Harris is the figure who is supposed to be a bridge for generations and the face of the party’s future. She painted a vision of unity and a call to action.
“Let’s fight with confidence in ourselves, and a commitment to each other. To the America we know is possible. The America, we love…..no matter what we look like, where we come from or who we love.”
But for a candidate who was lauded for her ability to “prosecute the case” against President Trump, she also made sure she left time to attack Donald Trump’s record and deliver thinly veiled digs like, “I know a predator when I see one.”
But the night will also be remembered by the excoriating critique of Donald Trump by Barack Obama who went further than ever before. His successor he said, had “no interest in treating the presidency as anything but one more reality show that he can use to get the attention he craves.
“Donald Trump hasn’t grown into the job because he can’t.”
In Detroit, Michigan, a critical swing state in this election, I did meet people who still have faith Joe Biden can grow in the job and a strong sense that he needs to.
At Livernois Bike Shop in Sherwood Forest, full with customers looking for ways to get outside in these coronavirus days, Ali Atit tells me he wanted Bernie Sanders, but is willing to get behind Biden “for the sake of change.”
Biden is an imperfect candidate for an imperfect age: a 77-year-old who has a complicated record when it comes to race.
At the Good Times bar down the road, there is fear Trump’s campaigning strengths and a silent majority could threaten Biden’s chances.
Victor Gibson, a former teacher, tells me he’s not convinced those who took to the streets to protest this year will also go to the ballot box. He believes young people have lost faith in politicians and when it comes to a result he says, “I think they’ll slip Trump back in.”
Democrats must have African-American voters on side to win. They are historically a loyal voting block for Democrats, but there’s a generational divide, and many feel their votes have been taken for granted. But, Biden’s best bet may be harnessing enough anger and exhaustion.
Sherry Gay-Dagnogo, the Detroit House Caucus chair, tells me: “I think the current president asked what do we have to lose. I think we’ve seen what we have to lose is humanity and respect.”
The group of friends have lost many family members to COVID-19 and view Trump’s handling of the pandemic as a huge motivating factor to vote, even if Joe Biden isn’t everything they want.
Brandi Neal, a government worker, says “Some of us will just have to hold out noses and clutch our pearls as we did it (vote). But we won’t survive another four years of this administration.”
Turnout will be critical.
I spoke to about 40 people in the bar. All said they would have their say this election. There is a sense of fondness and forgiveness about Joe Biden here. His political fortunes changed when African- American voters turned out in big numbers for him in South Carolina.
He needs to build on those votes and attract the next generation. That is no easy mountain to climb.