James Carville, the strategist who has been one of the few establishment Democrats to have been warning about Joe Biden’s age issue before the president’s disastrous debate performance Thursday, has called on his party to “deliver change” and replace the president as its nominee for November’s election.

In an interview Monday with the Guardian, he also said it would be in the US’s best interest for Biden’s Democratic presidential predecessors Barack Obama and Bill Clinton to help persuade him to half his re-election – and support an open nomination convention in Chicago in August to select a new ticket for the party.

“If it’s too hard for the Democrats to deliver change, then they’re going to hurt themselves bad – really bad,” said Carville, who led Clinton’s successful 1992 presidential campaign. “Change is messy. But you have to listen to the vox populi. We want something new. I see staggering talent in the Democratic party.

“Let them speak their minds.”

Carville’s comments come after he has spent months saying Biden was too old, should not run again – and that the party’s governors were among the most “breathtaking talent”, as he put it in a 16 December interview. He added: “We’re keeping it bottled up.”

Democrats initially did not like hearing Carville attack Biden on the age issue. Pennsylvania senator John Fetterman told Politico he wished Carville would “would shut the fuck up!”

Avoiding a tangle with Fetterman, Carville fell in behind Biden after his re-nomination became inevitable. But in February, when Biden refused an interview offer during the Super Bowl, Carville fumed on CNN: “It’s the biggest television audience, not even close, and you get a chance to do a 20, 25-minute interview … and you don’t do it, that’s a kind of sign that the staff or yourself doesn’t have much confidence in you.

“There’s no other way to read this.”

As Carville watched Biden debate Donald Trump in his room at the hotel where he was staying for the Ideas festival in Aspen, Colorado, the Democrat quickly began faltering. Text messages started arriving after three minutes.

“I wasn’t surprised this happened,” he said. “It’s like knowing where you were the night JFK got shot, you’ll remember where you were on Thursday night. Every election is about one thing: is this a change election, or a stay-the-course election? That’s the axis on which politics operates. This is clearly a change election.”

As the text traffic intensified, he got back to the Atlantic’s Ron Brownstein, saying: “I tried, man. I tried.”

He saw a theme emerge as people reacted to Biden’s raspy voice and confusion in certain answers, saying: “OMG – that was my daddy five years ago! Everyone understands old age as they go through it or help their parents through it. Everyone has helped an old lady across the street. This is a water-cooler issue.”

Carville turns 80 in October. “This is nothing to be ashamed of,” he said. “Ageing happens.”

But, Carville said, there are now “two wars going on”. On one hand, “the ugly, mad supreme court is greenlighting every element of corporate rapacity in their rulings trying to gut the EPA”.

The supreme court on Monday also issued a ruling granting “absolute immunity” from prosecution to presidents for acts that are considered official. Many interpreted the ruling as a boost to Trump’s efforts to combat prosecutions against him in connection with a sprawling effort to subvert the outcome of the 2020 election that he lost to Biden – all of which are still pending after his conviction in the New York case involving hush money paid to Stormy Daniels.

Meanwhile, Carville said, Biden is being told to “stay the course” by the first lady, Jill Biden, and their son Hunter, recently convicted himself – on federal gun charges.

“They have every reason in the world to love a tremendous dad, a guy who worships his wife,” Carville said. “My children love me, even though my oldest daughter says: ‘Daddy, I wish you wouldn’t say some of those things.’”

“Change is hard!” Carville declared in a voice of semi-thunder. “The polls show 72% of the public wants change – and not the kind Trump and his cult offer.”

Carville said organizers of an open convention could line up speakers including California governor Gavin Newsom, Michigan governor Gretchen Whitmer, former New Orleans mayor and Biden re-election campaign co-chairman Mitch Landrieu, Vice-President Kamala Harris, and Maryland governor Wes Moore.

“You know how many networks would cover that as major breaking news?” Carville said. “It could split the party, yes. Then go cut a deal! Soothe the hurt feelings – do the things that make politics what fascinates so many of us.

“It was a lot of trouble for Magellan to go around the world. Martin Luther King Jr had a nice church in Montgomery – it was a lot of trouble to lead that bus boycott.”