Barack Obama took to the stage for his final Democratic convention as a sitting president, saying he was "more optimistic about the future of America than ever before," and that Hillary Clinton was eminently qualified to be the next U.S. president.

While touting accomplishments at home and abroad with respect to health-care reform and climate change, Obama admitted there were several challenges confronting the country.

Obama, speaking exactly 12 years after he raised his national profile as a junior senator from Illinois with a rousing convention speech, said the country wasn't facing a typical election in November.

"There is only one candidate in this race who believes in that future, and has devoted her life to it; a mother and grandmother who'd do anything to help our children thrive; a leader with real plans to break down barriers, blast through glass ceilings, and widen the circle of opportunity to every single American ā€“ the next President of the United States, Hillary Clinton," said Obama.

Obama traced the evolution of his relationship with Clinton, including their bitterly-contested campaign for the 2008 Democratic leadership.

"She was doing everything I was, but just like Ginger Rogers, it was backwards in heels," he said.

Obama: 'Democracy isn't a spectator sport'1:54

The president said given her experience, from partner of former president Bill Clinton to New York senator to secretary of state in his administration, "there has never been a man or a woman more qualified than Hillary Clinton" to be president.

Obama took Donald Trump to task for his suggestion at the Republican convention last week he alone could solve the nation's challenges.

"America's never been about what one person says he'll do for us," said Obama. "It's about what can be achieved by us together, through the hard and slow and sometimes frustrating, but ultimately enduring, work of self government, and that's what Hillary Clinton understands."

After Obama's nearly hour-long speech, Clinton walked on to the stage to rousing applause from the crowd and a hug from the current president. 

Obama: Iā€™m ready to pass the baton'2:56

Obama followed the current vice-president, Joe Biden, and Biden's would-be successor, Tim Kaine, who took turns in attacking Trump's suitability for the presidency.

Biden told Democrats gathered for the party's convention that Hillary Clinton is the only candidate in November's election who understands the issues facing middle class families.

He praised Clinton as being smart and passionate, having known her for three decades, first as a rival politician's wife and then as a colleague in the Senate and in Obama's administration.

Biden, Kaine target Trump

Biden then turned his attention to Republican candidate Trump, setting up a refrain that would be repeated by the crowd.

"He has not a clue about what makes America great. Actually he has no clue, period," said Biden, leading to chants of "Not a clue" at Wells Fargo Center in Philadelphia.

Joe Biden: 'Trump doesn't have a clue'1:55

Kaine took the stage soon after to make his introduction to a national audience. The Virginian has served as a mayor, a lieutenant-governor and governor of his state, and is currently its senator.

"I humbly accept my party's nomination to be vice-president of the United States," Kaine said to loud cheers.

Kaine, who sprinkled Spanish and colloquialisms into his speech, touted his economic stewardship as governor during the recession and legislative response to reform gun laws after the Virginia Tech mass shooting in 2007.

He scored his biggest applause line by mentioning Trump's unwillingness to release his tax returns, a common practice in modern presidential campaigns.

"You cannot believe one word that comes out of Donald Trump's mouth," he said, prompting a reprise of the "Not a clue" chant.

Obama and others attempted to rebut Trump and the Republicans, who painted a dark portrait of an America under siege at their Cleveland convention last week.

Gun control

That theme was tackled early in night three of the convention by Connecticut senator Chris Murphy, who filibustered for 15 hours last month to agitate for gun control. Murphy, who has advocated on behalf of families of the children killed in a mass shooting at Sandy Hook Elementary School in 2012, said in his speech on the convention stage that Trump's comments and plans surrounding guns would lead to more carnage.

"I am furious that in three years since Sandy Hook, three years of almost daily bloodshed in our cities, the Republican Congress has done absolutely nothing to prevent the next massacre," said Murphy.

Chris Murphy: 'Demand change' on gun control0:37

Martin O'Malley, who ran for the Democratic nomination, was also a vociferous speaker in attacking Trump.

"I say to hell with Trump's American nightmare," O'Malley said. "We believe in the American Dream."

The third night of the convention came hours after Trump was encouraging Russian hackers to find missing emails from private servers Clinton set up, which resulted in an FBI investigation.

"No major party nominee in the history of this nation has ever known less, or has been less prepared to deal with our national security," Biden said.

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Delegates hold up signs during the third day of the Democratic National Convention at the Wells Fargo Center on Wednesday in Philadelphia. (Robyn Beck/AFP/Getty Images)

Leon Panetta, the former defence secretary and CIA head, contrasted Clinton's experience dealing with foreign threats and flashpoints with Trump's "erratic" statements.

"This is no time to gamble with our future," said Panetta, as some in the crowd chanted "No more war!"

John Hutson, retired admiral with the U.S. navy, brought up Trump's derisive comments to U.S. senator John McCain in 2015 about getting captured during the Vietnam War.

"Donald Trump isn't qualified to shine John McCain's boots," he said.

'Tug of war for America's soul'

Former New York City mayor Michael Bloomberg, the billionaire political independent, admitted he's disagreed with Hillary and Bill Clinton at times, but cast doubt on Trump's business acumen.

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

Rev. Jesse Jackson, in a speech early Wednesday evening, brought up the spectre of gun violence, imploring for a ban on assault weapons.

He characterized the election between Trump and Clinton as a "tug of war for America's soul."

"It's healing time, it's hope time, it's Hillary time," Jackson said in a refrain, his voice growing more forceful as he ended his speech.

'It's Hillary time'0:55

Testimonials also were given on gun control from families affected by the Charleston, S.C., church shooting in 2015, the Pulse nightclub shooting on June 12 and Gabrielle Giffords, the congresswoman who survived her injuries from a gunman in her own Arizona district.

The party also championed its devotion to environmental issues.

Karen Weaver, mayor of Flint, Mich., touted Clinton's commitment to "work for a lead-free America" as her city deals with an ongoing water crisis.

California Gov. Jerry Brown excoriated Trump for denying climate change and his state's water crisis.

"Trump says global warming is a hoax. I say Trump is a fraud," said Brown.

With files from The Associated Press