Biden also appeared uncertain when he spoke on The Earl Ingram Show, which is aimed at black listeners in Wisconsin but is also broadcast nationwide.
When asked why voting mattered, Biden delivered an answer that appeared to refer to the Supreme Court’s recent decision to grant Trump considerable immunity to prosecution.
He said: “You need someone, someone who is going to make sure that – the Supreme Court just issued a decision, by the way, that threatens the American principle that we have no kings in America. There’s no one above the law”.
Earlier on Thursday, Biden, 81, fluffed his lines when speaking to military families on the White House South Lawn.
During a four-minute address he said he was “not going anywhere” but he seemed to stumble when he referred to Trump as “one of our colleagues”.
The mistakes will only add to the pressure on Biden to step aside as the Democrat presidential candidate, as the party becomes more concerned about its ability to beat Trump in November.
While defenders of Biden say he has long mixed up words, during his 90-minute debate last week he frequently appeared lost.
Public support of Biden drops, trust in Kamala Harris rises
While he has admitted to supporters he does not speak as “smoothly” as he used to, questions as to whether he could serve another four years even if he manages to win have intensified. Polls show public support for Biden has dropped markedly since the debate, while trust in Harris has grown.
On Wednesday, Biden addressed Democratic governors, insisting that he had what it took to beat Trump and asking for their backing.
Yet the New York Times reported he had told them he needed to sleep more and work fewer hours. He said he ought not to hold events after 8pm.
While a handful of elected Democrats have called for Biden to step aside, most have held off.
There is speculation many are waiting for more detailed polls from battleground states that could emerge in the coming days for a clearer indication of how much support he has lost before speaking out in public.
On Friday, Biden will be scrutinised further when he sits for an interview with ABC News’s George Stephanopoulos that will air as a prime-time special.
Biden plans to be in Philadelphia on Sunday and hold a full news conference during the Nato summit in Washington next week.