According to Coke's website, "The Coca-Cola Company began its Christmas advertising in the 1920s with shopping-related print ads in magazines like the Saturday Evening Post. The first Santa ads used a strict-looking Claus, in the vein of Thomas Nast ... In 1931, The Coca-Cola Company commissioned Michigan-born illustrator Haddon Sundblom to develop advertising images using Santa Claus. For inspiration, Sundblom turned to Clement Clark Moore's 1822 poem "A Visit From St. Nicholas." Moore's description of the man as "chubby and plump, a right jolly old elf" led to an image of Santa that was warm, friendly and human. For the next 35 years, Sundblom painted portraits of Santa that helped to create the modern image of Santa." (More on Coca-Cola's Heritage pages).
Coca-Cola and Christmas (An article with Sundblom Santa images at the bottom)
The Coke Santas by and in the style of Haddon Sundblom (Scroll down for a montage of Sundblom Santas)
A Visit from St. Nicholas by Clement Clarke Moore on Oldpoetry.com