“Joe Biden is my brother,” Mr Obama said, on stage in Flint. “And he will be a great president.”
Mr Obama said he didn’t know Mr Biden that well 12 years ago, when he chose him as his vice president, but has grown to respect his “sense of self, his empathy and decency”.
He said: "For eight years Joe was the last one in the room when I made a decision, and he made me a better president. And he will be a great president.
“I never thought Donald Trump would embrace my vision. But I did hope, for the country’s sake, he might show some interest in the job, or he might take the job seriously. But he never has.”
Mr Obama accused him of “never taking the presidency any more seriously than a reality show, to give him the attention he craves.”
He referenced the Covid death toll, the unemployment level, and the fact that this week has seen soaring cases, even as the president continues to play down the pandemic and insist the US has “turned the corner”.
"His closing argument this week is that the media is too focused on Covid. And now he’s accusing doctors of over-blowing it, because they’ll make money off it.
“He cannot fathom that people would risk their lives to help others.”
Mr Obama repeated his criticism of the Trump administration’s handling of the pandemic.
“If we had handled the pandemic the same way as Canada, 140,000 fewer Americans would be dead,” he said. "Think about that.
“And yet, when asked whether he would do something different, he said: ‘Nope’.”
The Flint rally will be followed by an event in Detroit later in the day. Stevie Wonder will perform at the event in Detroit, designed to inspire and fire up Black American voters, who Mr Biden will need if he is to carry the state.
In 2016, the counties that include Flint and Detroit saw a significant drop in turnout, MI Live reported.
The overall Black voter turnout that year decreased for the first time in 20 years, according to the Pew Research Center.
A New York Times/Siena College poll released this week showed Mr Biden up eight percentage points in Michigan.
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