Democratic presidential nominee Hillary Clinton hugs US President Barack Obama as she arrives onstage at the end of his speech on the third night of the 2016 Democratic National Convention in Philadelphia, Pennsylvania, US, July 27, 2016. [Photo/Agencies]

PHILADELPHIA - US President Barack Obama painted an optimistic picture of America's future and offered full-throated support for Hillary Clinton's bid to defeat Republican Donald Trump 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 in a rousing speech that 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 on Wednesday night.

Hillary Clinton, the wife of former president Bill Clinton, will accept the party's White House nomination in a speech to end the convention on Thursday night. The election is on Nov 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.

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.

A former first lady and US senator, Clinton made history on Tuesday when she became the first woman to secure the presidential nomination of a major party. She will lead the Democrats against Republican nominee Trump.

Obama 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.

"Preach!" members of the crowd shouted. "Best president ever," someone screamed.

Obama listed what he described as progress during his two terms in office, such as recovery from an economic recession, the Obamacare healthcare reform and the 2011 killing of al Qaeda leader Osama bin Laden.

He said American values, not race, religion or political preference, were what made the United States great.

"That's why anyone who threatens our values, whether fascists or communists or jihadists or homegrown demagogues, will always fail in the end," Obama said.