Between 1980 and 2010 California’s health care policy field shifted from a business-dominated, closed-door pattern of decision making to an open political arena in which a wide-ranging and diversely resourced coalition advocating on behalf of beneficiaries had become an accepted partner in policymaking. This article examines this transformation, considering its broader implications for the political dynamics of the public-private welfare state and the role of advocacy groups in defending beneficiary interests. Our argument emphasizes coalition-building, probing not just which interests combine forces, but also showing how coalitions can expand over time and build their range of capabilities. We focus on three processes that build effective coalitions to influence public private policymaking: 1) an initial link that joins previously unconnected groups in umbrella organizations; 2) resource expansion that enlarges the engaged base by funding more diverse groups and expanding alliances with those organizations; 3) institutionalization of coalitional engagement by changing the rules of the game using such policy levers as regular hearings, provisions for participation, and transparency. We conclude by showing how these capabilities have positioned California to implement the Affordable Care Act and consider the implications for other states.