The deal is the latest in a flurry of mergers among health insurers that are scrambling to better position themselves in a tumultuous time for the industry.

Earlier in the week, the Justice Department stunned health care experts and even some Republicans by asking a federal appeals court to invalidate the entire Affordable Care Act. If successful, the lawsuit would largely dismantle the individual market and reverse Medicaid expansion in three dozen states.

House Democrats introduced a proposal on Tuesday to bolster the law through a series of provisions like increasing federal subsidies, but there is also talk among Democrats of adopting a much broader “Medicare for all” program. Some proposals supported by 2020 presidential candidates would do away with private insurers altogether.

“Everybody is trying to anticipate what the health care environment is going to look like in the next decade,” said Les Funtleyder, a portfolio manager at E Squared who owns shares of Centene. The premise behind expanding Centene’s foothold “is the government is going to play a role regardless of what happens in the U.S.,” he said.

The acquisition would help insulate Centene from the continued threats against the federal health law, Mr. Funtleyder added. “It almost doesn’t matter if the A.C.A. is repealed, because somebody is going to manage Medicare and Medicaid,” he said.

Unlike other health insurers, Centene has aggressively moved into the volatile Obamacare markets in various states. The company had already become a major presence in Medicaid through its purchase of other plans, like New York’s Fidelis Care.

Mr. Neidorff, who would remain as chief executive of the combined companies, played down the political risks facing the two companies. He dismissed the idea that the health care law would succumb to the current constitutional battle in the courts or to some Republican lawmakers’ renewed calls to repeal the law.