"The pandemic would have been tough for any president, we haven't seen something like this for 100 years," Obama said at a roundtable with Black community organizers in Philadelphia, where he was to hold his first public rally for Biden later Wednesday.
"But the degree of incompetence and misinformation, the number of people who might not have died had we just done the basics," Obama said.
"We can't afford another four years of this," he added. "We're in a deep hole."
While Obama is in Pennsylvania, Trump is to visit North Carolina, another of the battleground states which are crucial to victory on November 3.
A Quinnipiac University poll of likely voters released on Wednesday 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 he won the party nomination.
The 59-year-old Obama is to hold a drive-in car rally outside a baseball stadium in Philadelphia, the largest city in Pennsylvania.
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.
"If you do not vote you are not at the table," Obama told the community organizers.
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 upset victory over Hillary Clinton.
'Existential matter'
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.
Axelrod said Democrats had been strategic in having Obama make his first appearance on the campaign trail at this late date.
"It's been smart not to overuse him," Axelrod said. "They've been using him in targeted digital appeals to constituencies that Democrats need to arouse in this election: young people and people of color."
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 been a mix of optimism, telling Americans that the coronavirus outbreak is practically over, and 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.
"One of the things I love about Joe Biden -— he doesn't take on or talk about other people's kids," Harris said.
Trump held an outdoors campaign rally in Erie, Pennsylvania, on Tuesday and was to hold an event on Wednesday in Gastonia, North Carolina.
First Lady Melania Trump had been scheduled to join her husband in Erie but -- in a reminder of the pandemic which has cast a shadow over his presidency -- she cancelled at the last minute, complaining of a "lingering cough" following her own bout with the virus.
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.
Among those voting early was Utah Senator Mitt Romney, the only Republican in the Senate to vote to convict Trump at his impeachment trial.
Romney, in an interview with CNN, declined to say who he voted for but said he "did not vote for President Trump."
