Few British politicians have risen as far and as fast as Cameron. Elected to Parliament less than six years ago, still not widely known only two years ago and elected leader just last year, the Tory leader has become the hottest political property in the land–and he’s barely 40. And no single person is more responsible for his success than Hilton, the 37-year-old Oxford-educated son of Hungarian immigrants whose reputation as a political wunderkind is as high as his profile is low. He doesn’t talk to journalists on the record. His annual salary–reported to be £276,000–is guarded like a state secret. He doesn’t even have a job title. And yet, says one Tory insider, “if there’s one person Dave talks to, it’s Steve.”

Hilton’s elusiveness makes strategic sense. “He’s everything you wouldn’t expect a Tory to be,” says one political consultant, declining to speak on the record. Fashionably scruffy, often dressed in jeans and a T shirt under a shiny old leather jacket, he’s a stubble-bearded nail-biter who fizzes with ideas just like the advertising whiz he is in nonpublic life. But one thing is clear: he knows his job is to make Cameron shine, not himself–which is exactly what he’s done. With his help, Cameron has emerged over the past year or so as an immensely likable–and immensely credible–challenger to Prime Minister Tony Blair and his would-be successor, Gordon Brown (another nail-biter). Forget the Conservatives’ image as hard-nosed snobs. With Hilton’s help, they’ve become centrist tree-huggers with a social conscience–complete with a new green oak logo to replace the militant blue torch of old.

In some London political circles, there’s carping to the effect that Hilton’s rebranding has trended toward the gimmicky. For a while, Cameron cycled to Parliament–followed by a car ferrying his extra clothing and paperwork. But it would be a mistake to underestimate Hilton’s political savvy. This is a man, after all, who went straight from Oxford to Conservative Party headquarters, where he met Cameron and others who went on to found the Tory new guard. Among them: Hilton’s girlfriend, Rachel Whetstone, now a Google exec. (Hilton and Whetstone are godparents to Cameron’s first child.) In 1992, at 22, he worked as the Tory advertising coordinator in the party’s last national-election victory. At that point Maurice Saatchi, Margaret Thatcher’s favorite adman, hired Hilton away. “No one,” Saatchi said at the time, “reminds me as much of me when young as Steve [does].”

Hilton eventually left Saatchi’s firm to start Good Business, a corporate-responsibility consultancy. There the seeds of a new Conservatism were sown. “We’re pretty fed up with the idea that caring about people, that a belief in social justice and a desire for social progress, is something dowdy, dull and depressing,” he and the outfit’s cofounder, Giles Gibbons, wrote in their book, “Good Business.” Hilton is known for slick ads and focus groups, but in private he’s more likely to dwell on policy matters from climate change to early-childhood development.

So how conservative is Cameron’s éminence grise, really? A Labour Party rival was recently quoted as saying that Hilton told him he’d voted for the Green Party in 2001. A recent profile in the Daily Mail was headlined: is this the man wrecking the tory party? Some detractors go so far as to suggest that Hilton has latched on to the wrong man and the wrong party. The next British election, sometime in the next four years, should settle the argument–if controversy doesn’t bring down Hilton before then.