General Motors Co is looking for a new advertising agency for its luxury Cadillac brand as it tries to reverse a slide in sales in its home market.

Cadillac ads under the slogan Life, Liberty and the Pursuit had been handled by Modernista! since 2006.

The Boston-based firm, which also handles ad campaigns for broadcaster MTV and retailer the Gap, has also been the ad agency for GM's Hummer brand.

In a statement, Cadillac general manager Bryan Nesbitt said the company had begun a review process to look for a new agency. Modernista! had elected not to participate, he said.

Cadillac sales have dropped 44 percent this year, a worse performance than the 29 percent decline for Ford Motor Co's Lincoln unit.

Also on Friday, GM signed a deal to sell Hummer to China's Sichuan Tengzhong Heavy Industrial Machinery. The transaction still needs regulatory approval in China and the United States.

GM Vice Chairman Bob Lutz, who has taken charge of GM's marketing efforts, has said that he wants to see the automaker's post-bankruptcy advertising take aim at consumer reluctance to consider buying GM vehicles.