So the story goes, Moore created Constantine for two artists he worked with on Saga of the Swamp Thing. They were fans of the Police and wanted to draw someone who looked like Sting as part of the comic. The machiavellian magus with a working class background proved too cool to throw away and the character eventually landed his own series.
His genesis is given a wink by the later revelation that during his youth Constantine had been the leader of Mucous Membrane, a New Wave band.