At 82, Gore Vidal has reached an enviable position: he is an influential man of letters, a political activist, a scion of the New World aristocracy and a friend of the powerful and famous, including the Clintons.

So what does he think of Hillary Clinton’s stated intention to fight on to the bitter end for the Democratic presidential nomination? The reply is instant and searing: “I think her strategy is more or less insane.”

He continues: “I’d always rather liked her. She’s a perfectly able lawyer . . . But this long campaign, this daily search for the grail, has driven her crazy.”

In his view Barack Obama has won; and if the nomination is taken away from him, “I fear what our black population might do. There has never been a revolution of blacks – yet”.

During the Clinton administration, Vidal admired Bill’s understanding of the poor and of black people. His devotion to the Clintons has now been laid aside, however. By clinging on to her campaign, waiting for the small chance that Obama will make a terminal mistake, Hillary has crossed a line, he believes.

As for Obama, Vidal has taken time to warm to him. “I liked the idea of him, but he never managed to get my interest. I was brought around by his overall intelligence – specifically when he did his speech on race and religion.”

In Vidal’s opinion, “he’s our best demagogue since Huey Long or Martin Luther King”.

I ask if he thinks Obama has a similar charisma to that of John F Kennedy, whom Vidal got to know because he was related to his wife, Jackie.

“I never believed in Jack’s charisma,” Vidal says shortly. JFK, he believes, was “one of our worst presidents”; Bobby, his brother, was “a phoney, a little Torquemada”; and their father, Joseph, was “a crook – should have been in jail”.

So much for Camelot. “But Jack had great charm,” he adds. “So has Obama. He’s better educated than Jack. And he’s been a working senator. Jack never went to the office – he wanted the presidency and his father bought it for him.”

There’s no guarantee, of course, that the Democrats will triumph later this year, even if Obama does win the nomination. Does he think Obama can beat John McCain?

His views on the man the Democratic candidate will have to beat are even more brutal than his views on Hillary: “ You could beat McCain! I’ve never met anyone in America who has the slightest respect for him. He went to a private school and came bottom of his class. He smashed up his aeroplane and became a prisoner of war, which he is trying to parlay into ‘war hero’.”

In his view, McCain is “a goddamned fool. He was on television talking about mortgages, and it was quite clear he does not know what a mortgage is. His head rattles as he walks”.

However, in Vidal’s eyes, McCain is just a symptom of the real malaise affecting America today: the cynical subversion of the US constitution. “The Bush people”, he says, “have virtually got rid of Magna Carta and habeas corpus. In a normal republic I would probably have raised an army and overthrown them. It will take a hundred years to put it all back.”

By now he has worked himself up to a crisp fury: “Those neocons, lawyers, the big corporations – worse than that, extremists – want to get rid of the great power of oversight of the executive. See what they’ll try to do to Obama. They’re crooks. They’re just gangsters. They are the enemy of the United States. There’s no such thing as a war on terrorism. It’s idiotic. There are slogans. It’s advertising, which is the only art form we’ve invented and developed. It’s lies.”

Vidal has never been less than fully engaged with the politics of his country – but he seems angrier than I have ever seen him before. This may be because he has returned to live in the States only recently, after spending more than 30 years in Italy. He seems revived and refreshed by his furious reengagement with American politics.

For him, the biggest lie has always been to keep quiet; and the best life-enhancer is to provoke, unsettle, rile – in short, to make people face the truth. He remains a rarity.

The South Bank Show: Gore Vidal is on ITV1 tonight at 10.45pm