Gupte estimates that by 2020, half of the growing number of Medicare beneficiaries will be in Advantage plans -- some 38 million people in all. UnitedHealth has a similar outlook.

“There’s just a real strong overall value proposition with Medicare Advantage,” Steven Nelson, the CEO of UnitedHealth’s insurance operation, told investors on July 18. “We’re seeing that not only just with the folks that we serve, but as we talk to policymakers, too, there’s really strong support for it.”

Humana’s Medicare membership stagnated this year as the insurer pulled back from some markets and held benefits steady in an effort to improve profits. The effort succeeded in boosting earnings, and Humana said on Wednesday it plans to improve the appeal of its products for next year, boosting membership growth.

Startups Circling

As big insurers aim to expand their share of the market, investors have poured money into startups targeting Medicare Advantage. Clover Health, which offers the plans in New Jersey, raised $130 million at a $1.2 billion valuation in a recent funding round. Bright Health, which is making a big push into Medicare plans by teaming up with hospital systems, has raised a total of $240 million from investors.

Vivek Garipalli, Clover’s CEO, says the Medicare Advantage business model lets his insurer profit by taking better care of customers.

“You have a direct correlation with improving someone’s long-term outcomes and generating higher margins,” he said. “Our customers are with us for a long time.”

Garipalli said that his company won’t be selling Obamacare plans anytime soon. That’s in part because of the political threats the program faces, and because Obamacare customers switch plans so frequently.

“The exchange market, at least for Clover and the way we think about building our business, the churn is just really really high in individual insurance,” he said. “It didn’t really fit our model because it’s hard to build an outcomes-focused business.”

Despite the potential for growth and profits nationally, insurers have avoided offering Medicare Advantage plans in 147 counties across 14 states, according to an analysis from the Kaiser Family Foundation. Obamacare offerings are far more comprehensive -- 19 counties are at risk of having no insurer options next year.