Former US president Barack Obama has blamed his successor Donald Trump for the deaths of 170,000 people from the coronavirus pandemic in the United States and the millions of jobs lost in the ensuing recession.

On Twitter, Trump responded to Obama's appearance in all capital letters, suggesting Obama's decision to endorse Biden only after his Democratic rivals dropped out indicated doubts about Biden's candidacy

Obama, 59, himself a barrier breaker as the nation's first black president, warned that the country could falter if Trump is reelected, a stunning rebuke of the White House incumbent that was echoed by Kamala Harris at the Democratic National Convention on Wednesday night as she embraced her historic role as the first black woman on a national political ticket.

His assertion that Trump, a Republican, is incapable of meeting the demands of the presidency echoed the remarks from his wife, Michelle Obama, on Monday, that Trump "simply cannot be who we need him to be".

On Twitter, Trump responded to Obama's appearance in all capital letters, suggesting Obama's decision to endorse Biden only after his Democratic rivals dropped out indicated doubts about Biden's candidacy.

Throughout their convention, the Democrats have summoned a collective urgency about the dangers of Trump as president. In 2016, they dismissed and sometimes trivialized him. Now they are casting him as an existential threat to the United States.

Yet on the third night of the fourday convention, Democratic leaders also sought to put forward a cohesive vision of their values and policy priorities, highlighting efforts to combat climate change and tighten gun laws. They drew a sharp contrast with Trump, portraying him as cruel in his treatment of immigrants, disinterested in the nation's climate crisis and over his head on virtually all of the nation's most pressing challenges.

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Democrats demonstrated a hope that Biden, a 77-year-old white man, can revive the coalition that helped put Obama into office, with minorities, younger voters and college-educated women blunting Trump's lock on many white and rural voters.

The evening marked a celebration of the party's leading women, including remarks from Hillary Clinton, the first woman to become a major ticket presidential nominee; House of Representatives Speaker Nancy Pelosi and Massachusetts Senator Elizabeth Warren.

Harris, a 55-year-old California senator and the daughter of Jamaican and Indian immigrants, addressed race and equality in a personal way that Biden cannot when he would formally accepts his party's presidential nomination on Thursday night.

"There is no vaccine for racism. We have got to do the work," said Harris, her words emphatic though she was speaking in a largely empty arena near Biden's Delaware home.

"We've got to do the work to fulfill the promise of equal justice under law," the Democratic nominee for vice-president said."None of us are free until all of us are free."

Just 76 days before the election, Biden must energize the disparate factions that make up the modern Democratic Party－a coalition that spans generation, race and ideology. And this fall voters must deal with concerns over the COVID-19 pandemic that has created health risks for those who want to vote in person－and postal slowdowns for mail-in ballots, which Democrats blame on Trump.

Democrats hope that Harris and Obama, in particular, can help bridge the divide between those reassured by Biden's establishment credentials and those craving bolder change.

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The pandemic forced Biden's team to abandon the traditional convention format in favor of an all-virtual affair that has eliminated much of the pomp and circumstance that typically defines political conventions.

Heng Weili and Andrew Cohen in New York and agencies contributed to this story.