The push, which is the biggest to date for the title, launches this month and will comprise press, posters and radio.
Exact details of the creative are being kept under wraps, but The Spectator's publisher, Kimberly Fortier, said it would build on the magazine's "humour and intelligence". The campaign strapline will be "unashamedly elitist".
"The Spectator isn't dumbing down, it's for massively intelligent people. This campaign is aimed at people who we know will like it," she said.
Fortier added that the brief was not to try to find a new pool of readers, but to target bright 35-plus ABs who should be reading the magazine.
Johnny Hornby, a CHI managing partner, said: "This is a fantastic brand to be working on and a great creative opportunity."
The Spectator is owned by The Telegraph Group parent Hollinger, an existing CHI client. It was first published in 1711 and republished in 1828. It is the oldest continuously published magazine in the English language. It competes against titles such as The New Statesman and The Economist, and is edited by the MP Boris Johnson.
CHI recently unveiled a new campaign to support The Daily Telegraph's relaunch (Campaign, 28 February).
Last year, The Spectator won the contract to publish the Conservative Party's membership magazine, Heartland, after a pitch against an undisclosed number of agencies.
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