Americans are ready for a "better story" with Kamala Harris as their president, Barack Obama has asserted, as he made a forceful case for her, saying they don't want "four more years of bluster and bumbling and chaos" under her opponent Donald Trump.

Former president Obama, in his speech to the Democratic National Convention Tuesday night, remembered choosing his former vice president, Joe Biden, 16 years ago and recognised his friend’s accomplishments as Biden passed the torch to his running mate, Harris.

Harris, 59, is scheduled to formally accept the Democratic Party's presidential nomination on Thursday to take on Republican rival Trump, 78, in the November 5 election.

"America is ready for a new chapter. America's ready for a better story. We are ready for a President Kamala Harris. And Kamala Harris is ready for the job,” Obama said amidst applause from the audience.

Observing that history will remember Biden as a president who defended democracy at a moment of great danger, Obama, 63, said: “And I am proud to call him my president, but I am even prouder to call him my friend."

“History will remember Joe Biden as a president who defended democracy at a moment of great danger. I am proud to call him my president, but even prouder to call him my friend. Now the torch has been passed. Now it's up to all of us to fight for the America we believe in,” Obama, the first black American president, said in his prime-time address to the convention.

"And make no mistake: it will be a fight. For all the incredible energy we've been able to generate over the last few weeks, this will still be a tight race in a closely divided country - a country where too many Americans are still struggling and don't believe the government can help," he said.

Obama used the occasion to launch a scathing attack on Trump. He said that America does not need four more years of Donald Trump's presidency, slamming his policies and his character.

Obama said Trump’s move to kill a bipartisan border deal in the Senate and strip reproductive rights for women hurt Americans.

When the crowd booed, Obama 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,” Obama said.

He also levied personal attacks, calling Trump 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 are “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 to loud to cheers from the crowd.

“Donald Trump wants us to think that this country is hopelessly divided between us and them, between the real Americans who support him and the outsiders who don’t. And he wants you to think that you will be richer and safer if you just give him the power to put those "other" people back in. It's one of the oldest tricks in politics from a guy whose act has gotten pretty stale,” he said.

The truth is, Donald Trump sees power as nothing more than a means to his ends, Obama said.

"He wants the middle class to pay the price for another huge tax cut that would mostly help him and his rich friends. He killed a bipartisan immigration deal that would have helped secure our southern border because he thought trying to actually solve the problem would hurt his campaign. He doesn't seem to care if more women lose their reproductive freedoms since it won't affect his life,” Obama said.

Obama said Harris is a person who has spent her life fighting on behalf of people who need a voice and a champion.

“Kamala wasn't born into privilege. She had to work for what she's got, and she actually cares about what other people are going through. She's not the neighbour running the leaf blower. She's the neighbour rushing over to help when you need a hand,” she said.

Obama said Harris pushed his administration hard to get results for Californians when she was the state’s attorney general.

"As a prosecutor, Kamala stood up for children who had been victims of sexual abuse. As Attorney General of the most populous state in the country, she fought big banks and for-profit colleges, securing billions of dollars for the people they had scammed. After the home mortgage crisis, she pushed me and my administration hard to make sure homeowners got a fair settlement," he said.

Obama said the United States needs a president who cares about the millions of people all across this country who wake up every day to do the essential, often thankless work to care for the sick, clean the streets, deliver their packages and stand up for their right to bargain for better wages and working conditions.

For Trump and his supporters, freedom means that the powerful can do what they please, he said.

“We have a broader idea of freedom. We believe in the freedom to provide for your family if you're willing to work, the freedom to breathe clean air and drink clean water and send your kids to school without worrying if they will come home,” he said.

Obama said the rest of the world is watching to see if Americans can pull it off.

Obama said no nation, no society, has ever tried to build a democracy as big and diverse as that of the US 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. That’s why when we uphold our values, the world’s little brighter — when we don’t, the world’s a little dimmer, and dictators and autocrats feel emboldened, and over time we become less safe," he said.

“We shouldn’t be the world’s policemen and we can’t eradicate every cruelty and injustice in the world, but America can be and must be a force for good — discouraging conflict, fighting disease, promoting human rights, protecting the planet from climate change, defending freedom, brokering peace. That’s what Kamala Harris believes and so to most Americans,” Obama said.

“Whether you're a Democrat or a Republican or somewhere in between, we have all had people like that in our lives. People like Kamala's parents who crossed oceans because they believed in the promise of America,” he said.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.