WATCH LIVE: Joe Biden’s most important speech ever given during his decades in public service

Democratic White House candidate Joe Biden makes the most important speech of his long political career on Thursday as he accepts the party's nomination to take on Republican Donald Trump in an election taking place under the grim shadow of an unprecedented health and economic crisis.

"Donald Trump is not responsible for COVID-19 -- but he does bear full responsibility for the failed national response," Biden said ahead of his live television speech wrapping up the Democratic convention.

"We've got to hold him accountable this November," the 77-year-old former vice president and long-time senator from Delaware said in a tweet.

Biden, making his third White House bid after failing to win the nomination in 1988 and 2008, said he will use his speech to "discuss our plans to build back better and set this nation on a new path."

As Democrats prepared to wrap up their four-day convention, federal prosecutors announced the arrest of Trump's former top strategist Steve Bannon and a judge rejected the president's bid to block the release of his financial records to the Manhattan district attorney.

Trump described Bannon's arrest as a "sad event" before travelling to Pennsylvania to deliver remarks to supporters in the town of Old Forge near Biden's birthplace of Scranton.

Pennsylvania, like Wisconsin, is one of a handful of states seen as crucial to Trump's and Biden's hopes of victory on November 3.

Painting himself as the law and order candidate, Trump accused Biden of being a "puppet of the radical left" and attacked his son Hunter Biden's past business deals in Ukraine and China.

Trump was impeached by the Democratic-led House of Representatives in January for seeking political dirt on the Bidens from Ukraine but was acquitted by the Republican-majority Senate.

While his father was vice president, Hunter Biden served on the board of a Ukrainian gas firm accused of corruption but has not been accused of any wrongdoing.

He has kept a low profile during his father's campaign but is listed among those making remarks during the party convention's final day.

'Failure of leadership'

Because of the coronavirus pandemic, Biden will deliver his remarks to an audience consisting of only a few reporters, aides and family in his hometown of Wilmington, Delaware.

His acceptance speech will be the final salvo in a convention that was to have been held in the battleground state of Wisconsin -- but ended up being almost entirely virtual because of COVID-19.

In a show of unity, other speakers on the convention's final day include politicians who challenged Biden for the nomination -- former South Bend, Indiana, mayor Pete Buttigieg, New York billionaire Michael Bloomberg, entrepreneur Andrew Yang and New Jersey Senator Cory Booker.

On Wednesday, California Senator Kamala Harris made history as she accepted the nomination to be Biden's running mate.

Harris, the first black woman on a major party ticket, joined former president Barack Obama and Hillary Clinton, Trump's 2016 opponent, in denouncing the president.

"Donald Trump's failure of leadership has cost lives and livelihoods," Harris said. "We're at an inflection point."

Obama urged voters "to believe in Joe and Kamala's ability to lead this country out of dark times and build it back better."

On handing over the White House to Trump in 2017, Obama said 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.

Trump has left America's "worst impulses unleashed, our proud reputation around the world badly diminished, and our democratic institutions threatened like never before," Obama said.

Trump responded by telling reporters that Obama had been "a terrible president."

Bannon arrest

In New York, prosecutors announced the arrest of Bannon and three others for defrauding hundreds of thousands of donors in a Mexico border wall fundraising campaign.

The arrest is the latest in a string of high-profile criminal probes into Trump's inner circle which have seen the indictment or conviction of half a dozen of his close associates, including several key leaders of his 2016 campaign.

A US federal judge meanwhile refused Trump's effort to block the release of his financial records subpoenaed by a New York grand jury in a case focused on hush money payments and possibly bank fraud.

Judge Victor Marrero rejected Trump's argument that the demand for eight years of tax records from his accountants Mazars USA was too broad and constituted political harassment.

Manhattan district attorney Cyrus Vance opened a probe last year into Trump's payments in 2016 to allegedly buy the silence of two women who claimed to have had affairs with him.

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