Former President Barack Obama has appealed to Americans to vote for Kamala Harris, warning of a "tight" White House race between her and Donald Trump with less than a month to go.

At Thursday's campaign rally in Pittsburgh in the swing state of Pennsylvania, he urged the audience to "put down your phone...grab your friends and family and vote" for the current Democratic vice-president in the 5 November US election.

Obama also attacked her Republican rival Trump on various issues - including the economy and abortion.

Meanwhile, Trump on Thursday was campaigning in Detroit in another battleground state of Michigan, accusing the Joe Biden-Kamala Harris administration of "destroying our country".

The US presidential election is expected to be determined by a handful of states - including Pennsylvania - where both candidates stand a chance of winning.

Speaking at the University of Pittsburgh, Obama said: "This election is going to be tight because there are a lot of Americans who are still struggling out there."

"It's not just about policies that are on the ballot. It is about values," he added.

He denounced Trump as a selfish politician who only cared about "his ego and his money and his status".

"He's not thinking about you," he said, accusing Trump of "trying to tear down" the Affordable Care Act - key legislation that millions of Americans rely on.

The 44th US president and his wife Michelle Obama publicly endorsed Harris in July, shortly after Biden announced his decision to withdraw from the White House race.

Meanwhile in Detroit, Trump addressed a crowd for over an hour about a wide range of topics from voting rules to tariffs on China.

Much of his speech focused on his claims that the US auto industry is failing - a part of his broader message that the country's economy was suffering.

"They're destroying our country," he said of the Biden-Harris administration, accusing it of failing auto workers and claiming too many jobs have been moved to Mexico.

Also on Thursday, Democratic nominee Kamala Harris was in Nevada trying to secure the support of Latino voters.