Former US president Barack Obama excoriated Donald Trump and warned on Wednesday against complacency despite favorable opinion polls during his first public rally in support of Democratic challenger Joe Biden ahead of the November 3 election.

At the drive-in rally in Philadelphia, Pennsylvania, one of a handful of battleground states expected to decide the election, Obama lashed out at Trump's behavior and declared him "incapable of taking the job seriously."

But he also issued a stark reminder of 2016, when opinion polls showed Hillary Clinton as the clear favorite -- only for her and her supporters to be shocked by a Trump victory on election day.

Former Pesident Barack Obama remained on the sidelines during the Democratic presidential primaries but he threw his support behind Joe Biden after his former vice president won the nomination Former president Barack Obama Obama remained on the sidelines during the Democratic presidential primaries but he threw his support behind Joe Biden after his former deputy won the nomination Photo: AFP / Josh Edelson

"We can't be complacent. I don't care about the polls," the former two-term president told the rally outside a baseball stadium.

"There were a whole bunch of polls last time. Didn't work out. Because a whole bunch of folks stayed at home. And got lazy and complacent. Not this time. Not in this election."

He told supporters that too much was at stake to have four more years of Trump leading the nation, seeking to contrast his successor -- a Republican real estate mogul and ex-reality TV star -- with his former vice president.

US President Donald Trump at a campaign rally in Sunrise, Florida US President Donald Trump at a campaign rally in Sunrise, Florida Photo: AFP / MANDEL NGAN

"This is not a reality show. This is reality," Obama said.

"And the rest of us have had to live with the consequences of him proving himself incapable of taking the job seriously."

He pointed to Trump's running roughshod over previous norms, including his retweets of conspiracy theories, and accused him of mishandling the US response to the coronavirus pandemic.

Trump and Biden are to hold their second and final debate in Nashville, Tennessee Trump and Biden are to hold their second and final debate in Nashville, Tennessee Photo: AFP / JIM WATSON

"Our democracy's not going to work if the people who are supposed to be our leaders lie every day and just make things up," he said.

"And we just become numb to it."

Earlier, at a roundtable with Black community organizers in Philadelphia, he said "the pandemic would have been tough for any president, we haven't seen something like this for 100 years."

More than 40 million Americans have cast their ballots early in the presidential election, according to the US Elections Project More than 40 million Americans have cast their ballots early in the presidential election, according to the US Elections Project Photo: AFP / KAMIL KRZACZYNSKI

The coronavirus has killed over 220,000 people in the United States and seriously wounded the world's largest economy, prompting fierce criticism of the president's handling of the crisis.

"We can't afford another four years of this," Obama said.

Details of the second TV debate between US presidential candidates Donald Trump and Joe Biden on October 22, 2020 Details of the second TV debate between US presidential candidates Donald Trump and Joe Biden on October 22, 2020 Photo: AFP / Alain BOMMENEL

While Obama was in Pennsylvania, Trump visited North Carolina, another of the battleground states, where he riled up the crowd with popular campaign themes, such as his law-and-order platform.

"If Biden wins, the flag-burning demonstrators in the street will be running your federal government," Trump told spectators gathered at a municipal airport in the city of Gastonia.

US President Donald Trump speaks at a rally in Erie, Pennsylvania, a former Democrat stronghold that Trump won in the 2016 election. Trump's wife Melania Trump cancelled plans to accompany him to Pennsylvania at the last minute, ci US President Donald Trump speaks at a rally in Erie, Pennsylvania, a former Democrat stronghold that Trump won in the 2016 election. Trump's wife Melania Trump cancelled plans to accompany him to Pennsylvania at the last minute, citing ongoing discomfort following her bout with the virus. Photo: DC POOL / Saul LOEB

Biden is ahead of Trump by 2.3 percentage points in the state, according to a polling average by RealClearPolitics.

A Quinnipiac University poll of likely voters released on Wednesday meanwhile gave Biden a 51-43 lead in Pennsylvania, which Trump won by a narrow margin in 2016.

Trump is trailing Biden in the national polls, and another Quinnipiac poll spelled potential trouble for his reelection hopes.

The poll had the Democratic and Republican presidential candidates in a 47-47 dead heat in Texas, a state which Trump won by nine points four years ago and which hasn't voted for a Democrat since Jimmy Carter in 1976.

Biden, 77, had no public events on his schedule on Wednesday for the third day in a row, leading the 74-year-old Trump to accuse his Democratic opponent of going "into hiding."

The Biden campaign said he was preparing for the second and final debate against Trump in Nashville, Tennessee, on Thursday.

Obama remained on the sidelines during the Democratic presidential primaries, but he threw his support behind Biden after his former vice president won the party nomination.

The Biden campaign is hoping the star power of America's first Black president will help boost turnout among young voters and African Americans, who are key to Democratic hopes of recapturing the White House.

African Americans voted in record numbers for Obama in 2008 and 2012 but their participation dropped off in 2016, a contributing factor to Trump's victory over Clinton.

Trump has spent much of his first term in office seeking to erase the legacy of his predecessor but Obama doesn't view the election as a "personal grudge match with Trump," Obama's former chief strategist David Axelrod said.

"He views it as an existential matter for the country and for democracy," Axelrod told CNN.

While Biden has been keeping a low profile over the past few days, Trump is seeking to recapture the enthusiasm of four years ago with daily rallies in battleground states.

Trump's message has included telling Americans that the coronavirus outbreak is practically over, as well as attacks on the business dealings of Biden's son, Hunter, while his father was vice president.

Democratic vice presidential nominee Kamala Harris responded to the attacks on Hunter Biden on Wednesday at an early vote mobilization event in North Carolina.

More than 40 million Americans have already voted, according to the US Elections Project of the University of Florida, nearly 30 percent of the total turnout in 2016.