Obama to hold 'drive-in' rally for Biden in Philadelphia
Barak Obama (AFP)

Former US president Barack Obama's first appearance on the campaign trail for Democratic nominee Joe Biden will be a "drive-in car rally" in Philadelphia on Wednesday, the Biden campaign said.

Philadelphia, with a population of 1.6 million, is the largest city in Pennsylvania, a key battleground state in the November 3 election between Biden and President Donald Trump.

The 59-year-old Obama will "hold a drive-in car rally and encourage Pennsylvanians to make their plans to vote early," the Biden campaign said in a statement.

Democrats have been urging voters to cast their ballots early in the states that allow it because of the coronavirus pandemic and the potential for long lines on Election Day.

According to the US Elections Project of the University of Florida, more than 35 million Americans have already voted.

While Trump, 74, has been holding large campaign rallies around the country, Biden, 77, has opted to hold smaller events because of the health crisis.

The drive-in rally, where supporters remain in their cars, has become a feature of Biden campaign events.

Trump narrowly won Pennsylvania in 2016 but is trailing Biden in the Keystone State by 3.8 points, according an average of state polls by the website RealClearPolitics.

Obama remained on the sidelines during the Democratic presidential primaries but he endorsed his former vice president after he won the party nomination.

During the party convention in August, Obama urged voters to support Biden and said "our democracy" is at stake.

"(Biden) made me a better president," Obama said. "He's got the character and the experience to make us a better country."