Former US president Barack Obama has delivered a rousing and sometimes cheeky speech as he endorsed his long-time political ally Kamala Harris in her race for the White House.

Obama spoke to adoring crowds in a keenly anticipated prime-time address at the Democratic National Convention on Tuesday (local time), warning “the rest of the world is watching to see if we can actually pull this off”.

“No nation – no society – has ever tried to build a democracy as big and as diverse as ours before. One that includes people that, over decades, have come from every corner of the globe. One where our allegiances and our community are defined not by race or blood, but by a common creed,” he said.

Nearly eight years after the end of his presidency, Obama remains one of the most popular Democrats in the country, eclipsing Harris and the current administration, according to public opinion polls.

At 63, he is also keen to influence Democrats behind the scenes and maintains a legacy and a voice that loom large in moments of crisis.

Most recently, that was seen during the messy deliberations that ultimately led President Joe Biden – who was Obama’s vice-president – to step out of the race and endorse Harris, 59.

In his prime-time address on Tuesday, he brought his star power to a campaign, light on policy specifics, that has so far coasted on joyful vibes and the relief of Democrats delighted that Biden’s flailing campaign is over.

He said Harris and her running mate, Governor Tim Walz, embodied “a return to America where we work together and look out for each other”.

“If we each do our part within the next 77 days – if we knock on doors, if we make phone calls, if we talk to our friends, if we listen to our neighbours, if we work like we’ve never worked before, if we hold firm to our convictions – we will elect Kamala Harris as the next president of the United States and Tim Walz as the next vice president of the United States,” Obama said.

“Together, we too will build a country that is more secure and more just, more equal and more free.”

Like his successors, Obama presided over a closely divided country.

But his victories were enviable by today’s standards for a Democrat: He won in 2008 and 2012 by healthy electoral college margins, carrying not just Michigan, Pennsylvania and Wisconsin, but also Florida, Iowa and Ohio, states now considered out of reach for Harris and Walz.

But his presidency also fuelled the rise of Republican Donald Trump as a titanic right-wing figure.

Obama slammed Trump’s policies and character. When the crowd booed, he said: “Do not boo, vote.”

“We do not need four more years of bluster and bumbling and chaos. We have seen that movie before, and we all know that the sequel is usually worse,” he said.

He also got personal, saying Trump was a “78-year-old billionaire who has not stopped whining about his problems since he rode his golden escalator nine years ago”.

He said Trump’s complaints were “getting worse now that he’s afraid of losing to Kamala”.

“The childish nicknames, the crazy conspiracy theories, this weird obsession with crowd sizes,” Obama said. His last sentence was accompanied by a hand gesture that drew laughs from the packed crowd.

“America is ready for a new chapter. America is ready for a better story. We are ready for a president Kamala Harris.”

Obama was preceded on stage by his wife, Michelle, who offered a more measured speech. She urged Americans to vote for Harris and Walz because there is “no other choice”.

“As we embrace this renewed sense of hope, let us not forget the despair we have felt,” she said.

But she warned Democrats faced an “uphill battle” in the final months before the November 5 election.

“Kamala and Tim are doing great now. We’re loving it, they’re packing arenas across the country, folks are energised. We are feeling good. But remember there are still so many people who are desperate for a different outcome,” she said.

“No matter how good we feel tonight or tomorrow or the next day this is going to be an uphill battle.

“This is up to us, all of us, to be the solution that we seek. It’s up to us to remember what Kamala’s mother told her: ‘Don’t just sit around and complain. Do something’.”

Harris was not in Chicago on Tuesday. Instead, ahead of the Obamas’ speeches, she spoke to about 15,000 people in battleground Wisconsin in the same arena where Republicans held their convention last month.

She declared that she was running “a people-powered campaign”.

Meanwhile, Trump attacked Harris’s record on crime and safety, while brushing aside criticism from her campaign over holding an event in a Michigan town where white supremacists rallied a month ago.

The campaign stop was one of several Trump is holding this week in battleground states.

Harris and Trump are locked in a tight presidential race that polls suggest will likely be decided in a handful of battleground states.

-with AAP