A couple of weeks ago, the main cartoon in the London Evening Standard was a crude drawing of John Prescott's "battle bus". It was emblazoned with the slogan "Vote Labour or I'll hit you". I laughed all the way from London to Derbyshire. But the cartoon did not make me think any more deeply about the Deputy Prime Minister's suitability to head a great department, or the importance of politicians maintaining their self-control under all circumstances. It was just good, old-fashioned fun. Most cartoons aspire to be no more. It is hard to imagine one of the quickly completed drawings that illustrate some point from the day's news changing either a vote or an opinion.
They never did. In Victorian England, the golden age of political cartoons as well as of parliamentary government, the legendary cartoonists were doing exactly the job that their equally inventive successors do today - amusing rather than informing, emphasising an established point rather than breaking new ground. I own two or three of the original drawings that eventually found their way into Punch. Phil May's W E Gladstone (as an elderly Juliet) enticing Joseph Chamberlain (a reluctant Romeo) to his/her balcony raises a smile. But it did not change anybody's mind on Chamberlain's desertion of the Liberal Party because of his opposition to Irish Home Rule.
Most of the 19th-century Punch cartoons did not even aspire to amuse. Tennant's A Cabinet Notice is no more than the Grand Old Man, presumably anticipating a reshuffle, standing outside a theatre stage door. Pinned to the wall, a "Theatre Notice" announces cast changes. No doubt topical, but hardly a riot of wit or a feast of humour. Punch, like me, admired Gladstone to a point only just on the right side of the line that separates admiration from idolatry. So it was inclined to publish cartoons in which he was represented as a series of classical heroes in epic action. Perhaps he and his closest supporters were flattered as well as amused to see him in toga or Greek helmet. But I cannot believe that it enhanced his reputation with floating voters, any more than the clearly anti-Semitic portrayal of Disraeli - ringlets and shovel hat - did much to harm the Tory leader's standing.
The same must be generally true today. I am a Bell and Garland man - which sounds like a character in a Morris dance troupe but is, in fact, an enthusiast for the work of the principal cartoonists in the Guardian and the Daily Telegraph. Steve Bell draws what I think. John Major was (and no doubt still is) the boring sort of man who wears his underpants outside his shirt to make sure everything is secure and tidy. Margaret Thatcher has become (perhaps always was) a wild-eyed fanatic. And Tony Blair? Well, he smiles a lot. One of the cartoonists' most important functions is to confirm the prejudices that their newspaper encourages in readers. The Times's Richard Wilson and Peter Brookes constantly portray the Conservative leader as a bewildered and doomed child, reflecting, we assume, the view either of the Times readers or of its editor and leader writers.
Nicholas Garland is the most literary and (though he may not forgive me for saying so) the most intellectual of modern cartoonists. In the 1970s, when the Callaghan government was hanging on to power without a House of Commons majority, he drew the prime minister as a casualty of the Afghan war, with Thatcher, knife in hand, creeping up behind him. The cartoon was captioned with Rudyard Kipling's advice to Tommy Atkins:
When you're wounded and left on Afghanistan's plains,
And the women come out to cut up what remains,
Jest roll to your rifle and blow out your brains
An' go to your Gawd like a soldier.
Somewhere, a couple of years later, I wrote how that cartoon had caught the mood of Telegraph readers, who felt that Jim Callaghan was defying divine will by standing in the way of a Tory administration. Unfortunately, I misquoted the Kipling verse, writing "cut off" rather than "cut up". Garland wrote to me with a stern correction. Accuracy, he said, is very important in these matters.
Twenty years on, I remain awestruck by the seriousness with which he takes his trade. Last month, I opened an exhibition of "vulgar prints" - drawings quickly made and cheaply printed to commemorate great events (as well as public hangings) in the 18th century. Remembering that I had once written a biography of Nelson, Garland made me a copy of the print that commemorated the great sailor's death, and then asked a question of formidable detail about what appeared to be a trumpet in the hand of one of the minor characters. Drawing cartoons, if it is to be done well, is clearly not a frivolous business.
But the result, these days, very often is. History is full of cartoons that illustrate and illuminate great events. Dropping the Pilot, the commemoration of the Kaiser's foolish dismissal of Bismarck, was not meant to raise a laugh. "Very well then, alone" - the caption of the cartoon that depicted Churchill standing on the white cliffs of Dover immediately after France's capitulation - was intended to stimulate patriotic fervour. There were plenty of wartime cartoons of sailors on life rafts and airmen bailing out of damaged aeroplanes and, more recently, there have been illustrations of the horrors of the Balkans war and the inequalities of apartheid. But we think of cartoons as being, in essence, a branch of satire. So usually we expect them to make us smile, if not laugh out loud.
Once, they really influenced opinion. In The King of Brobdingnag and Gulliver, James Gillray shows George III holding a tiny Napoleon Bonaparte in his hand and frightening himself half to death by looking at him through a pair of binoculars. The message was clear. In order to continue his entirely justified vendetta against the king, Gillray understated what he knew to be a real threat from France. His constant assaults on the beastly habits of the French and his continual warnings about Napoleon's territorial ambitions prepared England for the rigours of war. Without Gillray the nation might have wondered: "Is it worth it?"
In this more sophisticated (and more sceptical) age, it is difficult to imagine a cartoonist turning Britain against support for President Bush's Star Wars project, or igniting a national campaign for the elimination of racism, poverty or even smoking-induced lung cancer. Cartoons make us glad. And nothing makes the sceptical British smile more than an assault on political pomposity. The bad news, for the iconoclasts, is that the politicians almost always enjoy even the most savage portrayal of their appearance and their follies. At least, most of them would prefer a rotten cartoon to none at all.
Cartoons - especially if it is assumed that the character is well enough known not to need identifying with their name on a briefcase - mean that a politician has arrived. When my job in the Cabinet was to prohibit price and wage increases, the Sun ran a regular feature of me as "The Caudillo". I wore jodhpurs and jackboots, a black shirt and the sort of hat with dangling tassels that was part of General Franco's regular uniform. I goose-stepped and gave the Nazi salute. After days of the creature appearing in the paper, I became fond of him.
Then I began to use cartoons as a barometer of my success. When I did badly, the wart just below the left corner of my mouth grew into a huge carbuncle and I was grotesquely fat. On the rare occasions I did well, the wart disappeared altogether, and I was pleasingly plump. Now I have disappeared, along with the wart. That I don't mind is an indication of how much my life has changed. It proves that I really have given up politics for ever.
"James Gillray: the art of caricature" is at Tate Britain (020 7887 8000) until 2 September. "Tabloid Culture: the popular print in England 1500-1850" is at the Whitworth Art Gallery (0161 275 7450), University of Manchester, until 24 June