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 joint 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 U.S. government health programs. Centene, which has roughly 14 million members, is focused on Medicaid and Affordable Care Act markets. WellCare also offers Medicaid and Obamacare coverage and has a large Medicare business; it counts about 5.5 million members overall, according to regulatory filings.

The first quarter has been the biggest of the past 12 years for health-care mergers and acquisitions globally, including biotechnology, according to data compiled by Bloomberg. Bristol-Myers Squibb Co.’s $89 billion purchase of Celgene Corp. including debt, accounted for much of the $173 billion total for 591 deals announced in the quarter so far.

Over the past few years, the health-insurance sector has consolidated rapidly, with many big players seeking what would once have been unconventional partners. CVS Health Corp., which runs a vast retail-pharmacy and pharmacy-benefit management empire, took over insurer Aetna last year, while Cigna Corp. acquired Express Scripts, a major rival to CVS’s Caremark unit. UnitedHealth Group Inc. has acquired a diverse array of doctors’ offices and surgery centers, among other businesses, while running huge insurance and drug-benefit arms.

The proposed combination of Centene and WellCare arrives just as President Donald Trump and Democrats in Congress are renewing a long-running feud over the Affordable Care Act. The law survived a 2017 repeal effort by Republicans, but 20 conservative-leaning states sued last year to have it repealed on the grounds that changes in tax law invalidated it. Trump’s Justice Department this week backed that effort, arousing the ire of Democrats.

On Tuesday, Trump tweeted that Republicans would be the “party of healthcare,” while Democratic leaders vowed to protect the ACA. House Democrats are also preparing to unveil legislation that would target high-cost drugs and protect people with pre-existing conditions.

The Affordable Care Act, now nine years old, has led to sweeping changes in the American health-care system that have spawned new businesses and reinvented old ones. Overturning the law entirely would mean undoing key planks including its Medicaid expansion, which has increased access to health coverage for millions of lower-income Americans.

An enlarged Centene could be threatened if higher courts uphold the request by the Republican-leaning states and the Trump administration to wipe out the entire Obamacare law, though legal experts have said that outcome is unlikely. The matter is currently before the Fifth Circuit Court of Appeals.

The deal will add to adjusted earnings per share in its second year, the companies said in the statement. Shares of WellCare climbed 12 percent to $258.83 in trading before U.S. exchanges opened. Centene fell 8.4 percent.

The deal could face antitrust scrutiny because of overlaps between the companies in a few states, including Missouri, Georgia, Illinois and Florida, according to Citigroup analyst Ralph Giacobbe. Cowen & Co. analyst Charles Rhyee said similar deals had been blocked in the past over concerns about insurers’ commercial and Medicare businesses, though he said that it is uncertain whether such concerns apply in Medicaid.

Allen & Co., Barclays Plc, Evercore Inc. and JPMorgan Chase & Co. are providing financial advice to Centene with Skadden Arps Slate Meagher & Flom LLP as legal counsel. Goldman Sachs Group Inc. is advising WellCare and Kirkland & Ellis LLP is legal adviser.