Joe Biden has been officially anointed the Democratic presidential candidate at the party's convention, helped over the line with some glowing testimonials from elder statesmen. Photo: Internet Source

Two Democratic former presidents, Bill Clinton and Jimmy Carter, and former Secretary of State Colin Powell, a Republican, endorsed Mr Biden.

Mr Clinton said President Donald Trump had brought "chaos" to the Oval Office.

Mr Trump trails Mr Biden in opinion polls ahead of November's election.

Mr Biden, the former vice-president under President Barack Obama, became his party's nominee on Tuesday night in a pre-recorded roll call vote from delegates in all 50 states.

This is Mr Biden's third White House bid, having formerly run in 1988 and 2008. The 77-year-old's campaign appeared to be in danger of collapse back in February this year.

On the second night of the party convention on Tuesday, with the theme "leadership matters," Mr Clinton delivered the key address.

"Donald Trump says we're leading the world," Mr Clinton said in his five-minute message pre-recorded from his home in Chappaqua, New York. "Well, we are the only major industrial economy to have its unemployment rate triple.

"At a time like this, the Oval Office should be a command centre. Instead, it's a storm centre. There's only chaos."

Following addresses from former First Lady Michelle Obama and Senator Bernie Sanders on Monday, Tuesday's speeches aimed to persuade voters the Democratic party is the best suited to repair problems at home and abroad.

Mr Powell said Mr Biden shared "the values I learned growing up in the south Bronx and serving in uniform".

The decorated four-star general said he supported him for president because "we need to restore those values to the White House".

In June, Mr Powell - who served under President George W Bush and has appeared at multiple Republican conventions in previous years - called President Trump a liar and endorsed Mr Biden.

He joins several Republicans who have endorsed Mr Biden, including former Ohio Governor John Kasich during the first night of the convention.