"Joe is just, look let's face it, he's been taken over by the radical left. He has no clue what they're doing," Trump told anchor Sean Hannity.
"They brainwashed him, he doesn't know where he is, he doesn't know what he's doing and our country will suffer.""Our stock markets will crash, bad things will happen," the president continued.
Trump added that Biden has made a "deal" with former White House hopeful Bernie Sanders, who is one of the most prominent progressive politicians in the country.
"He has been completely taken over, I think it's because he doesn't understand what's going on," Trump said of Biden, adding that the former vice president is "worse than Bernie Sanders at his best."
Earlier this week, Sanders and other Democrats on the so-called Unity Task Force released a detailed set of policy recommendations for Biden and the Democratic Party.
As Al Jazeera reported, Sanders and others called on Biden to adopt a slew of progressive policies.
The task force advised the former vice president to expand health insurance coverage, ban private prisons and undo Trump's immigration policies. The group also called on the Democratic Party's presumptive presidential nominee to push for criminal justice and education reform, and to introduce "New Deal style" economic policies.
As the publication noted, the task force's recommendations are not nearly as ambitious as policies Sanders and fellow progressive Elizabeth Warren advocated for during the Democratic primaries.
Still, Trump and his allies have alleged that Biden is moving to the left.
Elsewhere in the interview with Hannity, the commander-in-chief tried to cast doubt on Biden's fitness for office, alleging that the Democrat is cognitively impaired.
Trump repeatedly suggested that Biden is suffering a mental decline, challenging the former vice president to take a cognitive test.
Recent polling suggests that Trump is trailing Biden nationwide. According to a survey by The Hill/HarrisX released earlier this week, the Democrat enjoys the support of 43 percent of voters, while Trump is backed by 39 percent. In addition, the Rasmussen Reports' latest national poll showed the president trailing Biden by 10 percentage points.
In a recent interview, Trump appeared to acknowledge that Biden is ahead, but he nevertheless vowed to win in November.
"Now, I have to fight for the victory, but I've been fighting all my life," he said.