Veteran Washington journalist Carl Bernstein says those “very close” to Joe Biden have told him the “horror show” debate performance was not a one-off and that the US president’s cognitive decline has accelerated since the start of the year.

Biden’s halting and sometimes befuddled performance in last week’s debate has fanned concerns about his age and sparked criticism from donors, voters and some Democrats.

While some critics have pointed fingers at Biden aides who prepared him for the debate, others including The New York Times’ editorial board said Biden should step aside as the Democratic Party candidate against Donald Trump in the November 5 election. Biden and top Democrats say he has no plans to do so and intends to beat Trump.

Bernstein, the former Washington Post investigative reporter who rose to fame for his reporting on the Watergate scandal that eventually led to the resignation of Richard Nixon, says sources close to Biden have told him the president’s debate behaviour was not a rarity.

“These are people, several of them who are very close to President Biden who love him, have supported him and among them are some people who have raised a lot of money for him, and they are adamant that what we saw the other night, the Joe Biden we saw, is not a one-off, that there have been 15 to 20 occasions in the last year-and-a-half when the president has appeared somewhat as he did in that horror show that we witnessed,” Bernstein told CNN’s Anderson Cooper.

“And what’s so significant is the people that this is coming from, and also how many people around the president are aware of such incidents, including some reporters incidentally who have witnessed some of them.”

Bernstein, who did not disclose who he had spoken with, said that “particularly there has been a marked incidence of cognitive decline and physical infirmity”.

He said many of those who have witnessed the behaviour had approached Biden’s former chief of staff, Ron Klain, to tell him “we have a problem”, but they have been “pushed back repeatedly”.