The combined company will have about $30 billion in drug spending, making it a major player in the pharmacy-benefit management market.

Health insurer Centene Corp. agreed to buy managed-care provider WellCare Health Plans Inc. for more than $15 billion, expanding in the market for government insurance plans just as politicians in Washington appear to be gearing up for another health-policy clash.

Centene, based in St. Louis, offered $305.39 per share in cash and stock for Tampa, Florida-based WellCare, the companies said in a statement Wednesday. The boards of both companies backed the transaction, which has an enterprise value of $17.3 billion. Deal talks between Centene and WellCare were first reported by Bloomberg on Tuesday.

Both insurers have erected significant businesses around government health programs. Centene is focused on Medicaid and Affordable Care Act markets. WellCare also offers Medicaid and Obamacare coverage and has a large Medicare business catering to the elderly. A combined insurer would have 22 million members in government-sponsored health programs across 50 states, the companies said.