WASHINGTON (AP) — For more than 55 years, Medicare has followed a simple policy: covered benefits are the same, no matter if you're rich, poor, or in-between.

But as Democrats try to design a dental benefit for the program, one idea calls for limiting it based on income. The so-called "means test" is drawing internal opposition from many Democratic lawmakers, as well as advocacy groups for older people, like AARP.

Yet a senior Democratic congressional aide says an income limit is still in the mix as President Joe Biden tries to bring divided Democrats together on sweeping social and environmental legislation that would be their calling card in next year's midterm elections. The aide spoke on condition of anonymity to address internal deliberations. The White House did not respond to requests for comment.

Another Medicare alternative would involve charging upper-middle-class and wealthy seniors higher premiums for the new dental plan, an approach that's already applied to outpatient and prescription drug coverage and does not elicit such intense political opposition. It's not clear if Democrats are looking at that as well.

Medicare is the government's flagship health insurance program, covering more than 60 million seniors and disabled people. But it lacks dental, vision and hearing coverage, a gap that Vermont Independent Sen. Bernie Sanders has made it his mission to close. Committee-passed legislation in the House would incorporate the new benefits into the program.

But Biden and the Democrats are being criticized for spending too much money on their "Build Back Better" package, and particularly for providing child tax credits, educational and health benefits to people who could afford to pay their own way. The Medicare means test seems to have gotten its start with a set of policy talking points released earlier this year by centrist Sen. Joe Manchin, D-W.Va., representing his conditions for supporting the Biden legislation.

Under the heading "Families and Health," Manchin wrote "needs based with means testing guardrails/formulas on new spending." His office did not respond to requests for comment.

Meanwhile, progressive Sanders sees Medicare as the foundation for a future government health insurance program that would cover all Americans, regardless of age or income. His office had no comment on the income limitation controversy.

Means testing is getting some of its strongest support from the American Dental Association, which has called for a dental benefit that would cover seniors making up to three times the federal poverty line, or about $39,000, roughly half of Medicare recipients.

"We are not opposed to a Medicare dental benefit (but) we believe it should be focused on poor and low-income seniors," said Michael Graham, the group's top lobbyist.

Dentists are worried about Medicare setting fees for their services, as it now does with doctors. "Dentists need to be paid appropriately so they don't lose money," Graham said.

The dental association's stance drew a rebuke from AARP.

"It troubles us that groups like the American Dental Association, who you would think would want to get people dental coverage, are offering alternatives that would deny millions of people coverage," said David Certner, AARP's policy director.

"Entering a means tested benefit into Medicare is a nonstarter for us," added Certner.

Equally adamant is House Ways and Means Chairman Richard Neal, D-Mass., whose committee approved the Medicare benefits expansion. "I do not support any sort of Medicare means testing," said Neal, one of House Speaker Nancy Pelosi's top lieutenants.

As a general principle, Biden doesn't rule out income limits for certain government programs, says White House press secretary Jen Psaki.

"Means testing, I think, always has a bad connotation," Psaki said earlier this week. "And what we're really talking about here is a cap on income. So, we're talking about targeting and focusing the president's proposals, in some areas, on people who need help the most...and not providing this aid, assistance...to people who are in higher income brackets, who may not need that assistance."

However, there's no precedent within Medicare for an income-limited benefit, said health policy expert Larry Levitt of the Kaiser Family Foundation.

"The presumption has always been that having Medicare as a universal program creates political sustainability," Levitt said. "As soon as you start limiting Medicare benefits to only lower income beneficiaries, it could lose some of its political support."