Trending: When Daniel told Nick the magic had gone...

It was as much a gift for headline writers as an apparent wand in the eye for the Deputy Prime Minister. "Clegg-spelliamus!", said The Sun next to a picture of Harry Potter casting a spell on the strained face of the Liberal Democrat leader. Daniel Radcliffe, the boy wizard and emerging power in the fickle world of celebrity politics, had withdrawn his support for the Liberal Democrats. "Clegg's spell is broken," the headline read.

Radcliffe's words appeared to be wounding. "Nick Clegg has become a whipping boy," he said in an interview with Attitude magazine, then picked up by the red tops. "He has been totally used by the Tories... It's very unfortunate when you think how impressive he was in those pre-election debates. But he has made so many concessions."

The actor went further, not only dumping Clegg, of whom he said he was "rather fond" before the birth of the Coalition, but jumping straight into bed with Ed Miliband, the Labour leader. He becomes the latest high profile backer to quit the Lib Dem love-in, following the actor Colin Firth, Bella Freud, the fashion designer, and Kate Mosse, the author.

But how much was Clegg hurting as he turned to page three of yesterday's Sun? And what magic can a celebrity bring to a political party? If he was injured, Clegg is not admitting it. "We have nothing for you," a weary press officer says. "They'll be slightly embarrassed," suggests Mark Borkowski, the public relations expert. "But if you're a spin-meister at their little HQ, you're acclimatised to all this."

Andrew Hawkins, the founder of ComRes, the political polling company, says that, while Clegg's image could not be much poorer, any damage caused by Radcliffe's snub would only be temporary. "It could matter insofar as it might encourage people to think others are thinking what they're thinking," he says. "But beyond that I don't think Radcliffe or Colin Firth is going to be decisive in determining any one's future. And I'm not aware of a celebrity who has! had an effect on polls."

The era of the celebrity endorsement peaked after Labour's landslide victory in 1997, when the chief glad-hander, Tony Blair, hosted a series of receptions at No 10. Guests included Noel Gallagher, the poster boy for Cool Britannia, the culture Blair hitched to New Labour's brand juggernaut. But the Oasis guitarist later epitomised the potential weakness of such couplings. "A lot of us got carried away with the New Labour thing, me included," he said in 2002. "When Tony Blair was courting the music business, idiots like me thought we could have a say, but it became a publicity stunt on his behalf."

Last weekend, Gallagher completed a U-turn more violent even than Radcliffe's. "Under Thatcher, great art was made," he said. "There was a work ethic ... now, these kids brought up under the Labour Party ... it's like, 'Forget that, I'm not interested. I wanna be on TV'."

Whether such endorsements are sought by or, in the case of Radcliffe and the once-loved Lib Dems, given to a party, the collision of celebrity and politics can be risky. Borkowski says: "An endorsement can be a sugar rush but whether it's celebs popping into No 10 for a cup of tea or Jim Davidson backing the Tories [he appeared at the party conference in 2000], it can seem like a good idea at the time. But sooner or later one side falls out with the other."

Tracey Emin reportedly fell out with Labour after being dissed by Andy Burnham in 2009. The artist revealed last year she was a Tory voter. A valuable endorser must be committed. "You can't just do it if it seems good for your public image or to get some headlines," Borkowski adds. "It's about transparency and truth and if you're not really engaged it's going to be undone."

By Borkowski's standards, Eddie Izzard is perhaps the model celebrity campaigner. The comedian has fronted several election broadcasts for Labour and is one of its biggest donors. John Cleese filmed a three-minute monologue for the Liberal Democrats in 1992 and Sean Conner! y has be en a long-standing supporter of the Scottish National Party. The value of these endorsements, too, has not been measured.

Labour's new suitor may not have the same energy as Izzard. The party is coy about Radcliffe's switch. "It's always welcome to receive a pledge of support from celebrities as a way to raise our profile, reaching the wider public," a spokesman says. Miliband would do well to be cautious before sending film crews and roses to the Potter star's door.

Both Radcliffe (a film) and Gallagher (a single) have things to promote. Moreover, the words Radcliffe chose to endorse the party were less effusive than those he used to ditch Clegg. "From what I've seen of Ed Miliband, I really like him," he said. Potter's last spell does not look like his most powerful.