Former US president Barack Obama has made a passionate case against Donald Trump and in favour of Vice President Kamala Harris at an election rally.

Obama has been a vocal supporter of Harris since she ascended to the top of the Democratic ticket after US President Joe Biden stepped aside in July following a poor debate performance against Trump, the Republican former president.

Obama, whose White House term ended in 2017, is still popular with his party’s base.

The rally he headlined at the University of Pittsburgh on Thursday (local time) was held while Harris campaigned in Nevada and Arizona. It was the first of several events planned for Obama in battleground states in final weeks before the November 5 US election.

“I get it why people are looking to shake things up,” Obama told a crowd of supporters on Thursday.

“I understand people feeling frustrated, feeling ‘We can do better.’ What I cannot understand is why anybody would think that Donald Trump will shake things up in a way that’s good for you, Pennsylvania.”

Obama, who argued Trump cares only about his own ego and money, stressed that Harris was raised in the middle class and believes in American values.

“Kamala is as prepared for the job as any nominee for president has ever been,” he said.

As polls emerged showing Trump gaining favour among black voters, Obama said he also wanted to “speak some truths” to them.

“My understanding, based on reports I’m getting from campaigns and communities, is that we have not yet seen the same kinds of energy and turnout in all quarters of our neighbourhoods and communities as we saw when I was running,” he said.

He said the lack of enthusiasm for Harris “seems to be more pronounced with the brothers”.

“You are thinking about sitting out?” he said.

“Part of it makes me think – and I’m speaking to men directly – part of it makes me think that, well, you just aren’t feeling the idea of having a woman as president, and you’re coming up with other alternatives and other reasons for that.”

Obama is not the only former president the Harris campaign intends to deploy on the campaign trail.

Bill Clinton, like Obama a two-term Democratic president, and a former Arkansas governor, will make stops in Georgia on Sunday and Monday. He will then head to North Carolina for a bus tour later in the week to try to reach rural voters.

Young Americans are among a critical part of the coalition that the Harris campaign hopes will propel her to victory.

But voter registration among young people in 34 states is down compared with four years ago, according to data updated in September from the Centre for Information & Research on Civic Learning and Engagement at Tufts University.

The number of people aged 18-29 registered to vote in Pennsylvania in September was 15 per cent lower than it was on Election Day in 2020, the centre’s data showed.

Obama and his wife, Michelle, gave rousing speeches in support of Harris at the Democratic National Convention in August; the former first lady is also expected to campaign again for Harris.

Harris and Trump remain in a close race. Winning Pennsylvania could be key to winning the White House in a battle that will ultimately be determined by the Electoral College.

-with AAP