NEW YORK - In an electrifying speech, US President Barack Obama Wednesday night made his most forceful case yet for Hillary Clinton’s presidential bid, extolling the former secretary of state’s career of public service while declaring Republican presidential nominee Donald Trump as unfit for the presidency.

“There has never been a man or a woman, not me, not Bill (Clinton), nobody, more qualified than Hillary Clinton to serve as president of the United States of America,” Obama said as former President Bill Clinton looked on from his box along with thousands of party delegates attending the Democratic convention in Philadelphia.

After he concluded his speech wrapping up the third night of the convention, Hillary joined President Obama on stage to loud applause. The two -- the popular president and the presidential nominee who would replace him -- hugged and waved to the cheering delegates and departed with their arms around one another.

Despite a star-studded lineup of speakers that included Vice President Joe Biden and former New York City Mayor Michael Bloomberg and vice presidential nominee Tim Kaine, it was Trump who increasingly dominated attention at the largely-attended convention – in part because of his latest controversial comments urging Russia to unearth Clinton's missing emails.

“America is already great,” Obama said, referencing Trump’s campaign motto. “America is already strong. And I promise you, our strength, our greatness, does not depend on Donald Trump.”

The president described Hillary as “fit” and “ready to be the next commander in chief,” saying she is more prepared than any predecessor to occupy the Oval Office.

Along with his fulsome endorsement of Clinton, Obama offered a series of barbs for her opponent, departing from his usual practice and mentioning him by name a half-dozen times.

“Donald Trump calls our military a disaster,” he said. “He cozies up to Putin, praises Saddam Hussein. What we heard in Cleveland (Ohio) last week wasn’t particularly Republican – and it sure wasn’t conservative," Obama said of the Republican National Convention. "What we heard was a deeply pessimistic vision of a country where we turn against each other, and turn away from the rest of the world. There were no serious solutions to pressing problems – just the fanning of resentment, and blame, and anger, and hate. That is not the America I know."

By contrast, Obama said: "We don’t fear the future. We shape it; we embrace it, as one people, stronger together than we are on our own. That’s what Hillary Clinton understands – this fighter, this stateswoman, this mother and grandmother, this public servant, this patriot – that’s the America she’s fighting for."

The president’s speech followed a lengthy list of speakers who engaged in an hours-long take-down of Trump. Vice President Biden mocked Trump’s signature phrase from his reality television show to criticise him as having unbounded cynicism and lacking empathy and compassion.

“How can there be pleasure in saying, ‘You’re fired?”’ said Biden, 73. “He’s trying to tell us he cares about the middle class? Give me a break, that’s a bunch of malarkey.”

Earlier, Biden pronounced Trump less prepared for the Oval Office and the nuclear codes than any major-party candidate in history. "He has no clue about what makes America great," the vice president said. Then he ad-libbed: "Actually, he has no clue, period." The arena erupted into a chant, "Not a clue!""We cannot elect a man who belittles our closest allies while embracing dictators like Vladimir Putin," Biden said.

In a plain-folks manner that appeased some of Bernie Sanders' supporters – who continued to disrupt portions of the convention for a third consecutive night in defence of their left-wing agenda -- Kaine mimicked Trump as "a slick-talking, empty-promising, self-promoting, one-man wrecking crew."

"He never tells you how he's going to do any of the things he says he's going to do," Kaine said. "He just says, 'believe me.' So here's the question: Do you really believe him? Donald Trump's whole career says you better not."

Former Defence Secretary Leon Panetta lauded Hillary's participation in the operation that killed Osama bin Laden and lashed out at Trump's latest invitation for Russian intervention in the presidential campaign. "It is inconceivable to me that any presidential candidate would be that irresponsible," Panetta said. "In an unstable world, we cannot afford unstable leadership."

The third day of the Democrats' conclave also featured a guest appearance by Bloomberg, a political independent who denounced Trump's record as a Manhattan-based developer and urged delegates to vote for the "sane, competent person" in the race.

"Throughout his career, Trump has left behind a well-documented record of bankruptcies, and thousands of lawsuits, and angry shareholders and contractors who feel cheated, and disillusioned customers who feel they've been ripped off," Bloomberg said. "The bottom line is: Trump is a risky, reckless and radical choice. And we can't afford to make that choice."

By far the biggest speech of the night came from Obama, 12 years to the day after he electrified the 2004 Democratic convention in Boston with a keynote address that propelled him toward the White House four years later.

With an eye toward supporters of Bernie Sanders who have not warmed to Hillary during the long primary campaign or the sometimes divided convention, the president said: "If you’re serious about our democracy, you can’t afford to stay home just because she might not align with you on every issue. You’ve got to get in the arena with her."

His appearance, coming the night after former president Bill Clinton lauded his wife's career in public service, overshadowed the day's other major event: Kaine's official nomination as the party's candidate for vice president. Given Obama's popularity among the party faithful, Kaine was relegated to speak before him.

Trump was on the campaign trail Wednesday, telling reporters in Doral, Fla., that Obama is "the most ignorant president in our history" and adding that Hillary "would be even worse."

Donna Brazile, who took over as interim Democratic Party chairwoman after Debbie Wasserman Schultz stepped down over allegations that her staff favoured Hillary over Bernie Sanders, said she was not concerned by current polling numbers.

"Come Labour Day, you'll see Hillary Clinton pulling away again," Brazile said. "There is a path to the White House for the Democratic Party."

There was an air of confidence in the arena Wednesday as speakers and videos portrayed Hillary as a sound leader and Trump as uninformed and erratic. "This very morning, he personally invited Russia to hack us," retired Navy Admiral John Hutson said. "That's not law and order. That's criminal intent."

Hutson also noted Trump's attacks on Senator John McCain for getting captured in Vietnam, saying: "Donald, you're not fit to polish John McCain's shoes."

Unlike the Republican convention in Cleveland last week, most speakers in Philadelphia have addressed domestic policy issues. On Wednesday night, that included a lengthy diatribe against the gun lobby, led by survivors of horrific firearms attacks in Orlando, Florida, Charleston, South Carolina, and Newtown, Connecticut.

Delegates stood and applauded many of them, but none more than former Arizona congresswoman Gabrielle Giffords, still recovering from being shot in the head in Tucson more than five years ago. She appeared with her husband, retired astronaut Mark Kelly. “Speaking is difficult for me," Giffords said. "But come January, I want to say these two words: 'Madame President.'"