Arts: Viva Mexico

New Labour, new Britain, new era - new art? We don't expect so, not straight away, although this phrase has already appeared in print. It doesn't happen like that any more. At any rate, it would be hard to imagine Chris Smith, as new Secretary of State for National Heritage, saying something along these lines: "We proclaim that this being the moment of social transition from a decrepit to a new order, the makers of beauty must invest their greatest efforts in the aim of materialising an art valuable to the people, and our supreme objective in art is to create beauty for all, beauty that enlightens and stirs to struggle."

No, no remotely conceivable - and not only because the word "beauty" is no more likely to occur in a front-bench speech than the word "struggle", but because nowadays no government anywhere is likely to throw themselves into this kind of initiative. I'm not sure anyone would even like the idea of it. But this wasn't always so. It's been one of the big dreams of the century that governments on the left would make a working alliance with forward-looking art. It seemed right - though it's hard to say why - that progressive politics and progressive aesthetics should be on the same side.

It was an art-dream full of contradictory hopes - wanting to be innovatory and to be populist, wanting to be influential and to serve. And in actuality, attempts at this alliance haven't proved good. After the Bolshevik Revolution there was a short-lived collaboration between the Communists and the Russian Futurists, which produced some exciting works but ended in clamp-down and the dead hand of Socialist Realism. The Federal Art Project of Roosevelt's New Deal administrations was a happier affair. At least it kept many artists in work (another obvious attraction of this sort of project), but the results - mainly a plethora of folksy murals in post-offices - didn't really make the grade art-wise.

Only in one place, so the story usually goes, did the idea bear good fruit. In Mexico. It's the Mexican muralists who are the most serious contenders for great, state sponsored, left-leaning art. And the so-called "big three" of the muralists - Rivera, Orozco and Siqueiros - were among the signatories to the Manifesto of the Syndicate of Technical Workers, Painters and Sculptors that was quoted in the first paragraph. It was issued in 1922, shortly after the Mexican revolution had consolidated its hold on the country, and the artists got access to a lot of public walls.

They also made prints, and work by those three dozens of others are included in Mexican Prints at the Fitzwilliam Museum in Cambridge. There are over 100 pictures in the show, going up to the 1950s, and the majority take public-spirited, populist or propagandist themes: scenes of labour or revolutionary heroism or the capitalist and fascist menace.

I'm not quite sure about the Mexican thing. It certainly makes most other modern art contemporary with it look very tight-lipped, almost wilfully inexplicit. Conversely, the Mexicans get away with a lot that in (say) a European artist would be frowned on as much too cute or lurid. I'm not sure, that is, whether it quite earns the dispensation it generally received from normal modernist good taste. Is the idea that it's OK because it was in a good cause? Or because this is Mexico and different standards apply?

Diego Rivera, for instance: one of the main things that is attractive about his work is that it's cuddly. His compositions are made up of friendly, rounded forms - mostly human - overlapping and snuggling up together close. And this is somehow alright with him, whereas when you get a very similar thing in Stanley Spencer it's dodgy. Perhaps this because in Spencer there's always some private sex agenda at work but, in Rivera, it just reads as a public thing - solidarity, comfortably co-operative activity. They both treat people as soft toys, though.

It doesn't have to be people either. It can be done with landscape. There's a very Riveraesque image by David Alfara Siqueiros of the revolutionary leader Zapata, on horse-back. Behind him, the terrain picks up the shapes of the horse's head and rump in an arrangement of little hillocks - or they might be a camp of bivouacs - which fill the background with closely- packed humps. The effect is extremely homey, indeed womby. You get a like feeling in Luis Arenal's Sleep, which has a collection of people asleep in some sort of mass shelter, each one wrapped top to toe in a lovely fat blanket that folds round them like a bap or a pasty.

It would be good to know what exactly that scene was of. But the exhibition is, by current standards, distinctly under-curated, very low on contextual information, which is a pity, because many of the images evidently have some particular political occasion, and many of them appear to be aimed at a very general public, so it would do no harm to have a few details about the history and the channels of publication and distribution. The cursory brochure calls them "art for the people" but doesn't disclose how the people got to see them, or what matters were on their minds when they did.

Nor, when it comes to populism, are the meanings always as straightforward as you might expect. Jose Clemente Orozco treats the subject of people en masse in very contradictory ways. Rear Guard shows a rear view of some revolutionary militia, men and women, rifles at the shoulder, babies slung over backs, marching away from us with the heads bent into their sombreros that recede into the distance - forward, onward, together.

That's the expected message. But if you turn to his Zapatistas, communal action is now done as satire. You have another receding swarm of sombreros, shown from slightly above, so tightly gaggled that all you can see of most of them is the big hat, and the few faces that appear to have an expression of deranged glee, implying the same look under every hat and the absurdity of this proud collective identity. Or, in The Masses, he does collectivity as explicitly horrible - a sea of blabbering or gaping mouths and grasping hands that, if it's meant to signify universal crying, need looks indistinguishable from a show of ruling-class revulsion.

To complete this repertoire of response, you could add Isidoro O'Campo's Win a Million, which has a line of human shapes drooping along in front of a Lottery ad just being a drifting crowd of excludeds. Now none of these four images, except Rear Guard, is particularly well designed, but they each use the modernist device of repetitive semi-abstracted forms, and each one puts in or brings out the different content: heroic, ridiculous, bestial, hopeless. It's a good lesson in what more laconic modernists, using a similar vocabulary, deliberately weren't doing with it.

But the most shameless example of this sort of thing is seen in a couple of pictures by Jean Charlot, an emigre French artist who did much to get Mexican print-making going. They're both called First Steps, showing a mother getting a toddler walking, and one of them in particular demonstrates how the geometrization of the figure can be given the smoochiest turn. The mother is converted into a snug mushroom, her large head bent over head-on to make a semi-circle that is enclosed in the wider semi-circle of her shoulders. But these edges, which another artist might have made hard, are softened, and the whole body is a model of bungy compactness, cuddlier than anything in Spencer even. You hardly need the baby between her arms. She's a big baby herself, but doing a grown-up thing.

Stirring the struggle? It's figure without struggle, absolutely smothered with the artist's own affection. It makes you feel quite fondly towards some busting, strenuous Stakhanovite monster, swinging his hammer. But even if the whole big idea never came persuasively true anywhere in the world, you can't help wondering about it. What would New Labour art look like? Does the question have to be a joke?n

At the Fitzwilliam Museum, Cambridge, to 31 Aug (01223 332900)