Satanic imagery is a bit more complicated, actually, than a pair of red eyes behind a mask that might be the Lone Ranger's, and which are drawn more from B-grade horror movies than any European Christian tradition. But if the Conservative publicity machine is really trying to draw parallels between Blair and the original Devil, it should be careful.
By the end of the Middle Ages, there were three things that everyone knew about the Devil (apart from the fact that he was Bad). The first, which is obviously implied in the advert, is that he was the consummate liar and deceiver. But the others were that the Devil was enormously powerful and enormously sexy. Blair certainly wants to be the former, and probably wants to be thought the latter: it is surprising that the Tory party should so readily hand him such an accolade.
If the image is going to demonise Mr Blair, it will work as much around our subliminal expectations of Satan as around anything specific about the Labour Party, old or new. We can learn about what these might be by looking at the history of Christian iconography.
Demonising the opposition has long been a part of the political job. During the Reformation, pro-Catholic propaganda pictures of Luther frequently showed him either as the Devil or with the Devil sitting affectionately on his shoulder and whispering inspiration into his ear. The reformers responded with vicious caricatures of the personified Papal Bull, who is shown in 16th-century prints sitting at a table with a revoltingly obese Pope; the Bull's horns and tail were far more explicitly demonic than the red eyes and sinister mask that the Tories have pained on Mr Blair.
The Devil was strong and subtle. He started out as a mere snake but quickly took on all the powers of the dragon, breathing fire and wreaking havoc. As well as having immense physical strength, he also apparently had great mental powers as well. He could dispute with the greatest theologian. He was the master of disguise - dressing up as someone else being the very core of "lying", and one of the reasons why cross-dressing was more or less proof of heresy, as Joan of Arc discovered.
The Devil was sexy: he established his bond with witches by seducing them. The very word "glamour" derives from the spell that, under his auspices, witches performed: to "cast a glamour" was their crime. The word eventually extended to the alluring but delusive beauty that the witches gained from their association with the Devil.
Because people knew all this about him, they were far less worried by evil than we seem to be. If you believed in Satan you also believed in a God who had overcome Satan - and there were therefore simple ways of dealing with him. Quite simply, the Devil was terrified of Jesus, so you had only to name Christ or make the sign of the cross and the Devil would be forced to flee. (A good test, this. Make the sign of the cross when you next see Tony Blair on the TV, and if he explodes in a puff of vile smoke you should vote Tory next time. If he does not, you can safely follow your normal method of political discernment.)
The important thing in the Middle Ages was to keep the Devil in proportion. Hell was a real threat, and should not be ignored, but to let the Devil terrify you was equally reprehensible. In Christ there was nothing to fear - the Devil could be laughed at. Teresa of Avila recalls that when her visions began, she consulted a confessor about how she would know whether they were from God or Satan. He instructed her to greet their onset with a lewd gesture. If they were divine, her humility (in not assuming the visions were sent by God) would be pleasing to the Lord, and if they were from the Devil he would flee because the Devil, being unspeakably proud, cannot bear to be laughed at.
In the light of the victory of Christ, the argument went, the Devil should be mocked. The little red devils with horns, goats' feet and tails, familiar from mediaeval paintings, were a product of this attitude. When the Devil appeared in art there was often an element of farce. In the Mary of Nimmegen mystery play, for example, Satan confesses that all the devils are slightly deformed: "It is not in our power, we devils from hell/To incarnate ourselves... without some little defect here or there,/Be it in the head or the hands or the feet." Look at Hieronymus Bosch images to see the devil portrayed in innumerable playful forms.
What is frightening now is that we seem obsessed by evil without any counterbalancing sense of a triumph of good; or even any decent myth resources. We say we don't believe in the Devil, but popular culture is full of images that depend on his power: horror movies, invasive aliens, invisible diseases, ritual abuse, the overwhelming "evil" of a Hamilton or a West - a devil too powerful for us to ward off with simple charms, too amorphous for us to visualise, and too scary for us either to admit we are scared of or to laugh at.
What is worrying is that the creators of this new poster are aware enough of these lurking fears to try to exploit them. It is precisely to the point that there should be no actual, measurable or examinable message in these advertisements: they just say, "be frightened".
It must be quite hard for the other political parties to respond to this particular campaign. Either to ignore it altogether, or to join in, carries very real risks. If Labour can learn anything from history, its best bet would probably be to laugh at it. Might I recommend a poster showing Blair as a rather cherubic angel with non-aerodynamic winglets, still grinning sweetly, floating in the air with a slogan under his feet, "Labour rises above the slings and arrows of Tory abuse".