Former US President Barack Obama made a passionate case against Donald Trump on Thursday during a campaign rally in Pennsylvania in which he appealed directly to men to reject Trump's bravado and back Vice President Kamala Harris.

Obama has been a vocal supporter of Harris since she ascended to the top of the Democratic ticket after 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, held while Harris spent the day campaigning in Nevada and Arizona, is the first of several events he plans to do in the coming weeks in battleground states that are likely to decide the election.

In remarks that lambasted Trump both for his character and his policy proposals, Obama zeroed in on male voters, a constituency Harris has struggled to win over.

"I’m sorry, gentlemen. I’ve noticed this, especially with some men who seem to think Trump’s behavior, the bullying, and putting people down are a sign of strength. I am here to tell you that is not what real strength is," he said.

"Real strength is about helping people who need it and standing up for those who can’t always stand up for themselves. That is what we should want for our daughters and for our sons."

Before the rally, during a stop at a local campaign office, Obama went a step further, suggesting Black men were not supporting Harris because of her gender.

"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."

At the rally, Obama said Trump only cared about his own ego and money. He stressed that Harris was raised in the middle class and believed in American values, which he contrasted with Trump's mistruths and deliberate attempts to deceive, including recently about the government's response to hurricanes.

"When did that become okay?" Obama asked.

He highlighted Harris' "concrete plans" on housing and taxes.

"Kamala is as prepared for the job as any nominee for president has ever been," he said. "With Kamala, you've got actual plans. Trump - concepts of a plan." (AFP)