The private equity firm founded with a quarter-billion dollar check from the former Microsoft chief Steven A. Ballmer has made its first purchase.
The firm, the Stagwell Group, which is being run by the former advertising executive Mark Penn, will acquire SKDKnickerbocker, the public relations group best known for its advertising and campaign work for prominent Democratic clients.
It is the first step in what Mr. Penn said would be an attempt to create a cohesive group of advertising, research, public relations and marketing companies.
“The goal is to build a great group of companies that have a strong understanding of the digital world,” Mr. Penn said in an interview. “It’s a bit of a Noah’s ark, but we’ll have one of each kind.”
Mr. Penn said he also expected to acquire firms that specialize in digital design, Hollywood ventures, ad buying and financial communications.
Announced in June, Stagwell has $250 million in assets — most of it contributed by Mr. Ballmer, and some by Mr. Penn — and said it would use debt to strike deals worth as much as $750 million.