Speaking on Wednesday at a drive-in rally in Philadelphia on behalf of Biden, his former vice-president, and Democratic running mate Kamala Harris, Obama offered his fiercest critique yet of his successor. He took aim at Trump's divisive rhetoric, his track record in the Oval Office and his habit of retweeting conspiracy theories.

"With Joe and Kamala at the helm, you're not going to have to think about the crazy things they said every day," Obama said. "And that's worth a lot. You're not going to have to argue about them every day. It just won't be so exhausting."

Obama, who governed for two terms and remains one of the most popular figures in the Democratic Party, blasted Trump for his handling of the coronavirus pandemic, noting that the president himself had fallen victim to the virus.

"Donald Trump isn't suddenly going to protect all of us," he said. "He can't even take the basic steps to protect himself."

"This is not a reality show. This is reality," Obama said in a nod to Trump's past as a reality TV host. "And the rest of us have had to live with the consequences of him proving himself incapable of taking the job seriously."

Obama's appearance filled a gap left by Biden, who has stayed at home in Delaware since Monday for meetings and preparation for his second and final debate with Trump in Nashville, Tennessee on Thursday (1200 on Friday AEDT).

With a Reuters/Ipsos poll showing Biden with just a four-percentage-point edge in Pennsylvania, Obama warned Democrats against complacency.

"We've got to turn out like never before," he said. "We cannot leave any doubt in this election."

Americans are voting early at a record pace this year, with more than 42 million ballots cast both via mail and in person before November 3 on concerns about the coronavirus and to make sure their votes are counted.

Four years ago, Obama took part in a rally in Philadelphia with then-Democratic nominee Hillary Clinton the day before the election, only to see Trump narrowly take the state. The Biden campaign considers winning there a top priority.

In remarks at an evening rally in Gastonia, North Carolina, Trump briefly mentioned Obama, noting he had supported Clinton in her losing effort. "It was nobody who campaigned harder for Crooked Hillary than Obama, right?"

North Carolina is another battleground state where opinion polls show a tight race. Harris was also in the state on Wednesday to mobilise voters in Asheville and Charlotte.

Obama won North Carolina in 2008 but lost it in his 2012 campaign. Trump won it in 2016.

Trump argued that coronavirus-related restrictions were harming the state's economy and complained that Democrats and the news media were overly pre-occupied with the pandemic.

"All you hear is COVID, COVID," the president said. "That's all they put on because they want to scare the hell out of everyone."

Even though Wednesday marked Obama's 2020 campaign debut, his support has been essential for Biden. He has appeared at joint fundraisers with Biden and Harris, and his network of well-connected former aides has been instrumental in helping the campaign outpace Trump in bringing in donations.

Biden's team said Obama would campaign in Miami on Saturday for the Democratic ticket.

The last days of campaigning are taking place during a surge in cases of COVID-19 and hospitalisations in battleground states, including North Carolina and Pennsylvania but also Wisconsin, Ohio and Michigan.

Polling shows a majority of voters are disappointed in the way Trump has handled the pandemic, which he has repeatedly said would disappear on its own.

Australian Associated Press