Political art is in strong demand, especially when the images are attractive in themselves. Republican posters from the Spanish civil war, some of which are prominently displayed at the Art and Power exhibition, can sell for pounds 1,000, if they are in good condition and well-designed. General Franco may have had most of the military punch but posters of the Spanish nationalists, while selling well, go for less.
"Most people buy posters for the quality of the image," says Leslie Sherlock, a dealer in posters. But the propaganda posters of 1930s Spain combine looks, content and historical importance. Production was at its peak then, with colours over-printed so they had an almost three-dimensional effect.
"Spanish civil war posters are very saleable - nothing sells for less than pounds 700, except the Franco ones," adds Mr Sherlock. "Very few Russian Revolution posters turn up. The Russian political stuff that does appear is very worthy - women with muscles like all-in wrestlers. They might go for pounds 500 for an ordinary image, or pounds 1,000 for the better stuff."
Political propaganda has tended to lean to the left - there are more posters around arguing for socialism than capitalism. Labour's Forward the Dawn is Breaking posters, complete with marchers and banners from the 1920s, will sell for pounds 500 or pounds 600. Government posters from the Second World War can also attract good prices - Mr Sherlock has just bought one of the Dig for Victory posters, which he is now selling for pounds 150.
Posters thrive on scarcity value; their purpose is to be displayed and to persuade, not to have a long life. Those that have survived, and remain in good condition, have doubled in price over the past 10 years.
Another dealer, Tony Scanlon, says that the most exciting recent development has been the release of old Russian posters, particularly avant garde art from the late 1920s and early 1930s. Despite the enormous risk of hiding material denounced by Stalin as bourgeois, many of these survived and have started to be sold over here. These will often go for pounds 500 to pounds 700, with the occasional rare item from a big name, sought by galleries, selling for pounds 20,000. John Heartfield, a German communist artist who continued to work for some years after Hitler got into power, produced the posters that are likely to reach the highest prices today. Many are displayed in the Art and Power exhibition.
Mr Scanlon says that demand may develop for some more recent posters, particularly of Solidarity, the anti-communist movement in Poland. He says Cuban posters have also been available - often anti-American productions of the 1960s with a caricature of ex-president Lyndon Johnson dropping bombs. Typically they sell for pounds 80 to pounds 100.
The most sought-after posters are the propaganda weapons, some from wartime, others peacetime efforts, especially those inspiring revolutions. Some of the best examples can be found in the Imperial War Museum in London and in Manchester's National Museum of Labour History. Such shops as Vintage Magazines in London sell a range of these posters.
Dealers like Mr Scanlon and Mr Sherlock work from home, sending posters by mail, though they also sell at fairs, such as those of the Ephemera Society, whose members collect all sorts of things, from bustickets to the school reports of famous people. They also sell sporting and transport posters.
There are occasional auction sales of political memorabilia, including posters, but with a greater emphasis on porcelain, such as commemorative plates and teapots, and jugs in the often comic likenesses of political leaders. Buyers include MPs, and at one recent sale the House of Commons bought many of the best items for display in its lounges.
Busts of Gladstone and Disraeli are in particular demand, and Churchill is popular, although post-war leaders including Clement Atlee, Harold Wilson and Edward Heath have never generated much interest. "Quality is important but of primary importance is the subject - how important they are, how popular," says John Sandon, the director of Phillips's European ceramics department.
There is one post-war leader who has stirred the emotions and, love her or hate her, Margaret Thatcher's image is collected. The Spitting Image reproduction of her head as a demented teapot is avidly collected, although it is only about 10 years old. It will sell today for up to pounds 90. Earlier this year Tony Banks, the left-wing Labour MP, sold some of his political memorabilia at auction. It attracted considerable interest and buyers paid a premium for the items he had owned. His Thatcher teapot sold for pounds 200.
o Inquiries: Imperial War Museum, 0171 416 5211; National Museum of Labour History, 0161 228 7212; Leslie Sherlock, 01449 740397; Tony Scanlon, 0171 727 1594; Ephemera Society, 0181 450 9998; Vintage Magazines, 0171 482 5083.