Theatre: The Fix Donmar Warehouse, London

Sam Mendes began his reign at the Donmar Warehouse directing the British premiere of Assassins, Sondheim's musical take on the motivation of that rag-bag of people who have pointed guns at US presidents. He returns to American politics now with The Fix, a new musical - by unknowns, John Dempsey and Dana P Rowe - that traces the rise and fall of a young would- be senator, Cal Chandler, the scion of a dynasty not entirely distinct from the Kennedys.

The contrast between the two shows is instructive. The Sondheim takes material that it is potentially offensive to put into this genre and then disarms criticism with a biting demonstration that, far from being incongruous figures in a Broadway show, these twisted loners and thwarted idealists represent the parodic embodiment of the American musical's self-assertive values.

There's no equivalent irony in The Fix, which aims at very broad and battered targets (the amorality of the political machine; the obsession with image over substance; the links between politicians and the Mafia; the remorseless dynastic mill) and which justifies itself as a musical on the undeniable, but scarcely thought-provoking, grounds that politicans are natural showfolk.

Hence a score which, while basically rock-driven, is free to nod in any direction - macabre Vaudeville number; rousing clap-happy gospel; country 'n' western ballad, and so on. Where the musical eclecticism of Assassins makes the dramatic point that all the characters are trapped in one or other form of national costume, in The Fix it mainly serves to advertise Mr Rowe's ability to write blandly in several styles.

The proceedings begin with a suited chorus clustered around the flag- draped coffin of a senator who has died in flagrante with a receptionist. At the end, there's the same stage picture, only this time the stiff is that first senator's son, Cal (played by the talented, Hugh Grant-lookalike John Barrowman), who has died because, as state governor, he's taken on the Mafia in a drugs-busting programme.

In between, we see how Cal's cartoonily ambitious Rose-Kennedy-meets- Lady Macbeth monster mother, Violet (Kathryn Evans), and his not-just- literally twisted and crippled gay uncle (a splendid Philip Quast) package the reluctant boy for a political career. Soon, he has a wife he doesn't love, a mechanical ability to stir audiences with a formulaic I-see-the- future autocue oration ("You delivered that speech as well as anyone I ever heard," declares the uncle, quite unconscious of anything back-handed in that compliment) and is busy burying his fatigue and dazed disorientation in heroin addiction and a junkie girlfriend (Krysten Cummings).

Sam Mendes' production has a prodigious slickness, with its tightly drilled, stylised chorus (who play everything from ominously strutting mob men to microphone-waggling lobbyists), its dazzling lighting effects, its droll TV news inserts, and the hard fluency it achieves through the mini revolve and the conveyor belt stage at the back. At times, though, this technical tour de force comes perilously close to illustrating one of the ills pinpointed in the piece: the primacy of packaging over content. Dealing with a young man who determines to be individual rather than a type, the show itself has unresolved problems shifting between the satirically representative and the personally heartfelt. Loose ends and undeveloped strands abound.

The whole project is the outcome of a collaboration between the Donmar and Cameron Mackintosh. I wish this partnership well: the commercial and subsidised theatres should co-operate to create the best conditions for nurturing new talent. It might have been kinder, however, to tarry a bit longer with this show in the workshop.

To 14 June. Booking: 0171-369 1732