The advertising industry’s newest target: itself. Omnicom has agreed to buy rival Interpublic for $13 billion in an all-stock deal that would create the world’s largest collection of agencies by revenue.

Omnicom Group is in advanced negotiations to acquire direct U.S. rival Interpublic Group in a deal that will merge two Madison Avenue giants and fundamentally recalibrate the advertising industry as it grapples with the ongoing decline of many of its traditional practices.