Washington (AFP)

Former US president Barack Obama will bring his star power to Kamala Harris's election campaign Thursday in a bid to get out the vote in the critical swing state of Pennsylvania.

America's first Black president is hitting the campaign trail in the steel city of Pittsburgh a day after Harris's Republican rival Donald Trump charged through the must-win state.

The still hugely influential Democrat will be urging people to vote early in person or by mail as Harris looks to lock in as many votes as she can in a nail-biting race.

Trump rallied on Wednesday in President Joe Biden's childhood hometown of Scranton, Pennsylvania and will head on Thursday to the auto industry capital of Detroit in Michigan, another battleground.

Wooing blue-collar voters in the former coal mining town of Scranton, Trump vowed to "drill, baby, drill" for oil and assailed Harris on the economy.

Harris will head to yet another swing state, Nevada, to reach out to Latino voters but the White House said she would be kept informed throughout the day about Hurricane Milton.

The monster hurricane crashed into Florida late Wednesday with Biden warning that it could be the "storm of the century."

Obama's trip to Pennsylvania is the first stop in what will be a month of campaigning for Harris in the seven swing states where the 2024 election is likely to be decided.

The White House race remains neck-and-neck between Harris and Trump both nationally and in the battleground states, including Pennsylvania.

Harris's campaign is counting on Obama, 63, who was president from 2009 to January 2017, to mobilise Black and young voters as she seeks the edge on November 5.

'Everything he can'

But Obama's main message on Thursday will be to drive home the early voting message in an agonisingly close race.

Democrats have historically favoured early voting over Republicans.

Trump has meanwhile frequently lashed out against anything except on-the-day voting, repeatedly blaming mail-in ballots for his 2020 defeat by Joe Biden, which he still refused to accept.

The Republican himself has also sometimes called early voting into question, despite efforts by his campaign to promote it.

"President Obama believes the stakes of this election could not be more consequential and that is why he is doing everything he can to help elect Vice President Harris," Obama's senior advisor Eric Schultz said in a statement.

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

He portrayed Harris -- America's first woman, Black and South Asian vice president -- as the political heir to his own trailblazing path.

Obama led the crowd in chants of "Yes she can" -- a riff on the "Yes he can" chants from his own 2008 campaign -- but warned that 2024 would "still be a tight race in a closely divided country."

The ex-president has also pulled in more than $76 million for the Democratic ticket in this year's presidential race.

The ex-president endorsed Harris, 59, after Biden dramatically dropped out of the White House race in July.