US President Barack Obama's speech to the Democratic National Congress offered a striking contrast to the picture painted of America by the Donald Trump campaign at the Republican equivalent.

Far from being apocalyptically grim, it was positive and upbeat as he offered full-throated support for Hillary Clinton.

In a speech that electrified the Democratic National Convention he urged Democrats to enable Clinton to finish the job he started with his election nearly eight years ago. The speech capped a night when party luminaries took to the stage to contrast the party's new standard-bearer with Trump, whom they portrayed as a threat to US values.

"There has never been a man or woman, not me, not Bill – nobody more qualified than Hillary Clinton to serve as president of the United States," Obama said to cheers at the Philadelphia convention last night.

Clinton, the wife of former president Bill Clinton, will accept the party's White House nomination in a speech to end the convention on tonight. The election is on November 8.

"Tonight, I ask you to do for Hillary Clinton what you did for me. I ask you to carry her the same way you carried me," Obama said. When he finished, she joined him on stage where they hugged, clasped hands and waved to the crowd.

The two were rivals in the hard-fought 2008 campaign for the Democratic nomination. After winning that election to become America's first black president, Obama appointed Clinton his secretary of state and now looks to her to carry on his legacy. The Republicans have pledged to undo key markers of the Obama administration including his healthcare reforms, and will appoint conservative judges to the Supreme Court.

Republicans have painted Clinton as a Washington insider who would represent a "third term" for what they view as failed policies under Obama, elected to a second term in 2012.

Speaking to delegates, Obama offered an alternative to businessman Trump's vision of the United States as being under siege from illegal immigrants, crime and terrorism and losing influence in the world.

"I am more optimistic about the future of America than ever before," Obama said at the Wells Fargo Center, a basketball and hockey arena.

He took aim at Trump's campaign slogan and promise to "Make America Great Again."

"America is already great. America is already strong. And I promise you, our strength, our greatness, does not depend on Donald Trump," he said.

Democrats hope to capitalise on the divisiveness of Trump, who achieved the party nomination after a rancorous primary contest. However, recent polls show him leading Clinton slightly and party strategists know there is everything to play for.

Senior Democrats and former national security figures lined up earlier on Wednesday to describe Trump as unable to steer America through the dangerous waters of today's world.

By contrast, many prominent Republicans, alarmed by Trump's provocative comments on illegal immigrants and Muslims, were absent from the party convention that nominated Trump for the White House in Cleveland last week.

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

Trump, who has never held public office, offered his critics fresh lines of attack on Wednesday, urging 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. Clinton has said those emails were private.

Speaking to the convention, US Vice President Joe Biden said Trump was an opportunist who had no clue about how to make America great or to help working families.

Drawing chants of "Not a clue" from the floor of the convention, Biden took Trump, a reality TV host, to task for his trademark slogan, "You're fired."

"He's trying to tell us he cares about the middle class. Give me a break. That's a bunch of malarkey!" Biden said.

Clinton waged another hard-fought primary battle this year, beating off an unexpectedly strong challenge from the left by Bernie Sanders, a US senator from Vermont. A scandal before the party convention involving leaked emails showing officials who were supposed to be neutral had worked behind the scenes for Clinton led to bitterness and dissent among Sanders supporters, which Democratic leaders have sought to tamp down.

Additional reporting by Reuters.