ART / It's the way he tells 'em: The joke is not always on the enemy in a Barbican double bill of political art

THE BARBICAN is now showing a double exhibition of a political nature, and already it has run into a little trouble with officialdom. Downstairs, there is an excellent and comprehensive retrospective of John Heartfield: the inventor of political photomontage, a German despite his name, and most famous for the anti-Nazi pictures he made for Communist magazines in the Twenties and Thirties, before the Nazis drove him into exile.

The show has a lot of other work, such as book jackets and theatre designs made after the war, in East Germany, but it leaves Heartfield's reputation safely where it was. No trouble from this quarter.

Upstairs we come up to date with The Cutting Edge: a bright selection of contemporary British political cartoons, caricatures and art. There are graphic shafts from Trog, Peter Brookes and Steve Bell which it's good to see rescued from the daily death of newsprint. There is also a sculpture by Bill Woodrow, some pretty laconic photo-text pieces, and paintings by the group Art & Language, whose political significance is a secret known perhaps only to the artists themselves. And no trouble with any of that either, so far. No, it was the poster for The Cutting Edge, a photomontage designed by Ralph Steadman, that provoked the might of the armed bourgeois state, in the form of London Underground, which refused to exhibit it. Too much blood on it.

Perhaps this has its consolations. What you lose in publicity, you gain at least in the kudos of censorship. On the other hand, if you're looking to get censored, then London Underground is absolutely the first place to apply. The Underground is painfully aware of how sensitive its customers are, and compensates for stationary escalators by operating a scrupulously anodyne display policy, which has seen off many much milder images. They might have had a case, though, on artistic grounds. Steadman's poster is almost a parody of what one expects from 'political photomontage' - a melange of guns, barbed wire and cut-up bodies, with the picture participating vigorously in the atrocities it wants to indict.

This has been a weakness in much of Steadman's recent work. It is hoist on its own explosive agonising. It was some time ago that the artist realised that the most important political fact about any situation was his own wounded conscience, and introduced a vein of expressionism which tells you over and over how horrified Steadman is, and little else. His montages in response to the Gulf war - gateaux of rough-cut media images layered with gouts of red paint - are just school exercises 'on the theme of violence'. The botching is presumably meant to signify sincerity, but he still draws well when his conscience permits.

Even Peter Kennard's photomontages, although they are always more sober, are moving in this direction. Perhaps it is sheer exasperation at the world's refusal to see reason, when the artist has pointed it out so often: ingenuity doesn't seem worth the candle now. At any rate, in his Gulf pieces, the vocabulary of skeletons, missiles and burning globes is used so baldly that diminishing returns set in. But you need to keep your nerve here. The real force of political photomontage doesn't come from shocking juxtapositions - cries of protest - however much the medium encourages this. It works best as a confident display of moral superiority. The montageur plays a pictorial practical joke on the enemy, shows him up.

This really is Heartfield's lesson. It might sound odd to call him a practical joker, but his approach is generally cool. He is rarely overtly appalled. The ingenuity with which he works over the personalities and symbols of Nazism shows the artist and his cause in command of proceedings: it's good propaganda, because you feel it's on the winning side, even if the visible victories are only on paper. But it's important to see each work as a response to its occasion: Reichstag Fire, rearmament, a specific policy move or election result.

If the show has a fault, it is being a bit perfunctory about this context. The artist comes over too triumphantly - JOHN HEARTFIELD everywhere in bold type, when he wasn't on the winning side. You need more of the world in which the montage intervened - other posters, newspapers, photo-journalism - and which it failed to win over. Otherwise it becomes a timeless emblem of anti-Nazism.

That it has outlived its context has some irony. The work has lasted chiefly because Hitler has lasted. The power of montage is drawn from the power of the images it twists, and the image of Adolf Hitler has not faded. (Imagine Heartfield as a Spanish Republican artist dealing with General Franco: the case would be significantly altered.)

It may be that Heartfield never fully reckoned with Hitler's enormity. He doesn't try to buck up his own righteousness by making his enemy into a monster. He portrays him as a contemptible creature, the tool of capitalism or militarism, a small-time chancer and a rabble- rouser. Today this can look like a perilous misjudgement. But Heartfield wasn't producing memorial images of Hitler to hang in a late 20th-century art gallery. He was engaged in a tactical operation, reacting fast, trying to swing votes. And tactically the ploy makes sense, even if it didn't succeed. Compare that with the recent monstering of Margaret Thatcher, which often made her presence seem only the more mighty and inevitable.

Go back upstairs to the Spitting Image exhibit and you see something of this kind. It is a life-size tableau of Thatcher and her cabinet as Leonardo da Vinci's Last Supper. 'Thatcher thought she was the Messiah, but most of her disciples turned Judas' is roughly the message. But the physical size and presence of the piece, and the labour involved, are quite out of proportion to its (once) topical point. It would be fine as a drawn cartoon, but it aspires to be a grand memorial, both to Thatcher and to the mileage Spitting Image made from her. Rather a vain work, it has incidentally also run into trouble, with accusations of blasphemy from certain bishops. The point, however, is that politically, it is much too reverent.

Barbican, EC2 (071-588 9023), to 18 Oct.