Democratic presidential hopeful Pete Buttigieg sells his health-care plan as a moderate alternative to Medicare-for-all — offering a government-sponsored plan to those who want it while letting others keep their private and employer-sponsored insurance.

But the South Bend, Ind., mayor’s plan has a catch: If you choose not to enroll in any coverage, you could still be on the hook for thousands of dollars. Under Buttigieg’s proposal, Americans who lack coverage would be automatically enrolled in the government plan, potentially saddling them with a big bill at the end of the year for “retroactive” coverage.

Buttigieg’s campaign has said the proposal will ensure the candidate’s promise of universal health coverage — without eliminating private insurance, as some of his more liberal opponents have proposed.

But critics say the policy represents an expanded version of one of the least popular aspects of President Obama’s 2010 health-care law: the individual mandate, which fined Americans for not having health insurance. The mandate was repealed by the 2017 Republican tax law and ruled unconstitutional last week by an appeals court. None of the other leading Democratic hopefuls have reprised it as part of their health-care plans.

“Mayor Buttigieg’s retroactive enrollment is just a supercharged version of the unpopular individual mandate that he’s trying to obscure with misleading rhetoric,” said Matt Bruenig, head of the People’s Policy Project, a left-wing think tank.

“Instead of paying a $695 fine at the end of the year for being uninsured, you are hit with a bill to pay an entire year of premiums that could be ten times that amount,” he said. “This will be a political nightmare.”

A Buttigieg campaign spokeswoman denied the candidate had misled voters and said the potential back payments are justified because those making them are effectively insured throughout the year. The choice Buttigieg sells to voters, the campaign said in a statement, is not between having insurance or not, but rather where to get that insurance.

Buttigieg’s approach is part of his broader attempt to appeal to moderate voters who have helped propel the mayor to the top tier of the Democratic presidential field, particularly in the early-voting states.

Buttigieg has criticized Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.) for backing Medicare-for-all, which would require more than 150 million Americans to switch from their existing private health insurance plans to a new single-payer system, which could cost more than $30 trillion over a decade.

Buttigieg markets his $1.5 trillion plan as a more centrist and less expensive approach in part because it offers patients the freedom to choose between a government plan and a private insurer, rather than forcing all Americans onto the government plan.

“We must step up and deliver Medicare for all who want it while preserving your freedom to decide whether you want it,” Buttigieg said at Cornell College earlier this month.

“You take a version of Medicare, a public plan. We make it available to every single American. But I’m going trust you to decide whether you want it,” Buttigieg said in Davenport, Iowa, also earlier this month. “You can stay on a [private] plan if you’d rather. Or you can come to the public plan if you want.”

Buttigieg’s health plan has gotten less attention than the Medicare-for-all proposal supported by Sanders and Warren but may garner further scrutiny in the coming weeks as voters seek more information about the moderates vying for the Democratic nomination for president.

Buttigieg, former vice president Joe Biden and Sen. Amy Klobuchar (Minn.) have all released policies that would create a new government insurer — a “public option” — that would compete with private health insurance.

Though Biden has publicly said he would bring back the Obama­care mandate, it is not in his campaign’s official health-care plan. Biden’s plan would automatically place people in government insurance, but only if they are low-income and qualify for free coverage, like Medicaid.

Buttigieg’s plan aims to achieve universal insurance by automatically enrolling everyone without insurance in government plans.

Buttigieg’s campaign has not identified how it envisions charging Americans who do not make their premium payments, but the campaign has said that all premiums would be capped at 8.5 percent of income. It is unclear how many people could be affected by retroactive enrollment under Buttigieg’s plan, and the campaign did not provide an estimate. The Buttigieg campaign said “one option” for those who do not pay their health premiums is to charge these people through their yearly tax filings. The campaign also said low-income people would qualify for federal subsidies to reduce the cost of their health-care payments.

Before its repeal, the Obamacare mandate hit people without insurance with a $695 annual fine or a charge worth 2.5 percent of their income, depending on which was higher. Critics say Buttigieg’s plan could leave people with year-end bills of more than $7,000.

“The ACA’s mandate was never an actual mandate — it was simply a penalty if you didn’t have coverage. This is really a mandate to have coverage,” said Larry Levitt, a health-care expert at the Kaiser Family Foundation, which studies health-care issues.

Jared Bernstein, a former economic adviser to Biden, said these kinds of penalties are necessary to achieve universal coverage. Bernstein noted that without the retroactive penalty, consumers would have no incentive to sign up rather than simply wait to become sick before seeking care.

“You’re eligible for a penalty, which makes sense in my view because otherwise you have a free-rider problem,” Bernstein said.

“If you want to cover everybody — and Pete’s plan gets awfully close — you need a big enough risk pool to control costs. And to control costs, you need two things: a mandate and to subsidize people who can’t afford the costs. . . . That kind of mechanism is integral to the architecture of any plan that gets to universal coverage.”

Others who have studied the policy disputed that it resembled the ACA mandate.

“In the ACA mandate, you had to pay something for nothing. You’re paying a penalty at the end of the year. This isn’t that. . . . You are getting the benefit of health insurance and walking around with financial protection in the event of illness and injury,” said Christen Linke Young, who has written extensively about retroactive taxation for the Brookings Institution, a nonprofit policy group. Young isn’t on the campaign, but aides consulted her research as they crafted Buttigieg’s policy.

When Republicans challenged the mandate as unconstitutional in 2012, the Supreme Court upheld the policy in a 5-to-4 decision. Chief Justice John G. Roberts Jr. was the deciding vote as the court ruled the mandate qualified as a tax and is therefore constitutional.

Conservatives say Buttigieg’s revival of a similar idea would probably encounter fierce opposition, much like that expected for Medicare-for-all. The Kaiser Family Foundation said a majority of Americans “consistently” viewed the mandate unfavorably.

“If you are enrolled in a health plan against your will and face government penalties for it, that is essentially a non-choice single-payer plan for those people,” said Brian Riedl, a tax expert at the Manhattan Institute, a libertarian-leaning think tank. “There is no difference.”