US Vice President Joe Biden accused Republican presidential nominee Donald Trump of being an opportunist who hasn't a clue about how to make America great.

Speaking at the Democratic Party Convention on Wednesday night to endorse Hillary Clinton as the party's presidential nominee, Biden took aim at Trump's trademark reality TV slogan, "You're fired", saying the billionaire businessman's "cynicism" was unbounded.

"Think about that," Biden said. "How can there be pleasure in 'You're fired'?"

"He has no clue about what makes America great. Actually, he has no clue - period."

Biden said Trump was unfit to be elected to the White House on November 8.

"The threats are too great, the times are too uncertain, to elect Donald Trump as president of the United States," he said.

Grabbing the spotlight at a news conference in Miami earlier in the day, Trump urged Russia to find and release tens of thousands of emails that Clinton did not hand over to US officials as part of a probe into her use of a private email system while she was secretary of state.

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump told reporters.

Former CIA Director and Defense Secretary Leon Panetta told the Democratic convention that Trump had sided with Russia by asking for their services in hacking Clinton's emails, which should be enough to disqualify him from ever becoming commander-in-chief.

"It's inconceivable to me that any presidential candidate would be this irresponsible," he said.

Clinton's campaign quickly responded to Trump's statement, calling it the "first time that a major presidential candidate has actively encouraged a foreign power to conduct espionage against a political opponent.

"This has gone from being a matter of curiosity, and a matter of politics, to being a national security issue," Clinton's campaign said in a statement.

Trump steals the show

Al Jazeera correspondent Kimberly Halkett said the Democratic Party was able "to pivot" and attack Trump on his request for Russia to hack Clinton's emails.

"But at the same time, Donald Trump is also benefiting from this," Halkett said at the convention.

"Once again he's stolen the show. We are talking about Donald Trump again. And when we are talking about Donald Trump we are not talking about the Democratic convention," she said.

"The Democrats just can't seem to win ... they want to be talking about issues. They want to distance themselves from the Republican convention of a week ago and show how they are different. The problem is this email controversy, that seems to cloud Clinton's campaign, just won't go away."

In his speech to the Democratic convention, businessman and former New York mayor Michael Bloomberg said people needed to unite to defeat Trump, who he called a "dangerous demagogue".

"I'm a New Yorker and I know a con when I see one," Bloomberg said of Trump.

"The richest thing about Donald Trump is his hypocrisy."

The Clinton campaign has sought to portray Trump, who has no experience in public office, as temperamentally unfit for the White House.

They have contrasted Clinton's long experience and foreign policy skills with Trump's maverick position on international relations, which Clinton campaign chair John Podesta has described as "unsteady, unfit and dangerous".

Trump has proposed temporarily banning Muslims from entering the country and building a wall on the border with Mexico to stop illegal immigrants.