Gordon Brown has been offered advice on keeping jokes fresh, after repeatedly telling the same anecdote at hustings.
Mr Brown has told the same joke repeatedly
The prime minister-in-waiting has good timing but "sounds bored with the story from the moment he starts telling it", stand-up comedian Paul Sinha said.
But the joke, about Ronald Reagan and former Swedish premier Olof Palme, is "pretty weak", he told the BBC.
The Conservatives say Mr Brown has told the joke on at least eight occasions, along with other repeated anecdotes.
As the "iron chancellor" Mr Brown was sometimes described as dour and sombre, but more recently appears to have made a conscious effort to present a lighter image.
Although he is now the sole candidate to succeed Tony Blair, he has joined deputy leadership candidates at hustings and trade union events around the country in the run-up to a special Labour conference on 24 June.
At each event he has been telling anecdotes, including the Reagan joke which he usually begins by explaining "I was giving this story when I was doing a meeting the other day".
Mr Sinha told BBC Radio 4's PM programme that was his first mistake.
"That really takes away some of the magic and makes the audience feel a lot less special," he said.
Stand-up comedians also repeated the same joke at different venues, but the trick is to get a good night's sleep, so as not to go onto autopilot and start to make mistakes, he said.
The anecdote relates to a meeting between Ronald Reagan and Olof Palme, in which President Reagan asks whether Mr Palme was a communist.
An aide explains: "No Mr President, he's an anti-communist", to which President Reagan barks back: "I don't care what kind of communist he is".
Mr Sinha added that while Mr Brown did not appear to have been "born with funny bones", it should not stop him being able to tell good jokes.
But he added, Mr Brown's material let him down: "It's a pretty weak joke on every level ... it's not going to get him very far on the comedy circuit."