Pittsburgh (AFP)

Former US President Barack Obama lashed out at "crazy" Donald Trump Thursday and urged voters to back Kamala Harris as he brought his star power to the 2024 election campaign trail for the first time.

As he hit the stump in the must-win state of Pennsylvania, Obama also chided Black male voters for what he called hesitancy in supporting Democrat Harris because they "just aren't feeling the idea of having a woman as president."

Obama trained his fire on Trump during a pumped-up rally in Pittsburgh, comparing the Republican's long speeches to late Cuban communist leader Fidel Castro's and calling the billionaire out of touch with ordinary people.

America's first Black president admitted that "this election's going to be tight" as many voters were still struggling with high prices.

But he told the crowd that "what I cannot understand is why anybody would think that Donald Trump will shake things up".

The popular Democrat called Trump "crazy" to describe the 78-year-old former president's embrace of conspiracy theories.

As the crowd booed Trump, his successor in the White House, Obama added: "Don't boo -- vote."

"Kamala is as prepared for the job as any nominee for president has ever been," he added.

'Got a problem'

Vice president Harris's campaign said Obama's appearance, the first in a series in battleground states before the November 5 election, was designed to get people out to vote in crucial Pennsylvania.

Obama took aim at male voters who might be attracted by the Republican's appeals towards machismo.

"I'm sorry gentlemen, I've noticed this, especially with some men who seem to think Trump's behaviour, the bullying and the putting people down, is a sign of strength," he said.

"And I am here to tell you that is not what real strength is."

Earlier, in a surprise stop before the rally at a campaign field office in Pittsburgh, Obama made an unusually direct appeal to Black men, whose support polls show Harris has struggled to mobilise.

Saying he had some "truths" that he wanted the Black community to hear, Obama said that "you're coming up with all kinds of reasons and excuses, I've got a problem with that."

"Because 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."

Harris was in battleground Nevada for a town hall hosted by the Spanish language network Univision Thursday and later spoke at a rally in Arizona aimed at reaching out to Latino voters.

When a woman asked Harris at the town hall to name three of Trump's virtues, she replied: "I think Donald Trump loves his family, and I think that's very important.... But I don't really know him, to be honest with you. I don't have much more to offer you."

In Arizona, Harris addressed the devastation caused by Hurricane Milton in Florida, saying the federal government "has mobilised thousands of personnel" to recover and rebuild the region.

The White House said Harris had also taken part in a virtual briefing on Milton, which has sparked a political storm between Republicans and Democrats.

'Dumber than hell'

Trump was in the hotly contested state of Michigan on Thursday, unveiling new details of his protectionist plans for the US auto industry, including sweeping tariffs on vehicles not made in America.

Trump also ramped up his personal attacks on Harris, branding her "dumber than hell," and assailed the auto industry capital Detroit itself as run down as he was speaking to the city's economic club.

"Our whole country will end up being like Detroit if she's your president," he said.

Harris meanwhile said she had accepted an offer for a CNN town hall on October 23 in Pennsylvania, after Trump turned down a final televised debate with her.

"I think it's a disservice to the voters," Harris said in Arizona about Trump rejecting a second debate. "I also think it's a pretty weak move."

Democrats are hoping Obama could give Harris a boost in a race that has been locked with Trump for weeks, after her initial boost in the polls after she took over from President Joe Biden as the party's nominee in July.

Obama and former first lady Michelle Obama delivered rapturously received speeches backing Harris at the Democratic National Convention in August.