In the chorus numbers of this 90-minute musical, 11 singer-dancers fill the small stage, weaving between each other in intricate patterns or else, in the best of Teddy Hayes's 15 songs, standing almost to attention in an angled line, taking turns to step forward and sing lyrics that begin: “I am a farmer/a cleaner/a daughter...” and so on. The title of this song is We Are the People, and for the first time in the evening we catch a sense of that urgent, nationwide, class-wide gathering that brought Barack Obama to the White House.

The pity is that this outburst of sincerity is the closing number and, while some of the songs that precede it contain enjoyable and even impressive moments, others are too reliant on nonsense and the characters just too insubstantial to care about. A show that mixes music with comedy can be enormous fun, but when the comedy droops into silliness the fun fades.

Hayes, the writer as well as composer, sets his show in an Obama campaign office at a time when the arrival of Sarah Palin as John McCain's running-mate sends into reverse Obama's lead in the national polls. After an opening number with the chorus wearing pig masks and Palin in a pig costume (because of an Obama gibe), we are introduced to the panicking volunteers as they come up with wild suggestions for boosting the Democrat vote.

Any group of enthusiasts will contain the occasional freak but in this collection anyone marginally rational is the oddity. A spiritualist breezes in and out; Republican moles are unmasked and set upon; the flower-power girl has been time-warped from Hair. It's ridiculous, and irritating because of chances missed to give the hard-working cast more substantial characters. Each is given a sort of identity but little credible interaction occurs.

A welcome exception happens in He's a Negro, sung by two ex-Republicans, one black, one white (Rian Perle, Jamie McMillan), who reveal converging explanations for switching loyalties. But a trick is missed by not acknowledging that one of Obama's strengths is that he is half black, half white. Again, when the old campaigner (Jackie Skarvellis) recalls campaigns long ago, her lyrics namecheck small-town contests where merry cracks at every president since Eisenhower would have given us something to enjoy.

Tim McArthur, the director, allows too much unnecessary crisscrossing of the stage but as the choreographer he achieves some agreeable effects that compensate somewhat for disappointments elsewhere. Box office: 020-7704 2001, to March 21

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