They overwhelmingly voted for Barack Obama in 2008 and 2012. In return, Obama gave millennials the Affordable Care Act — something that 57 percent of them disapprove of, according to a new poll from the Harvard Institute of Politics.

They’re showing that disapproval with their checkbooks.

Only 28 percent of adults under the age of 34 have enrolled in Obamacare insurance policies.

Only 28 percent of adults under age 34 have enrolled in Obamacare insurance policies. Sixteen percent of people ages 25 to 34 have chosen to absorb the penalty for refusing insurance instead of trying to leverage heavy premiums and unrealistic deductibles.

Related: There is No ‘Fix’ for Obamacare

But since the younger, healthier generation isn’t signing up, insurance companies are drowning in mammoth pools of sick, underprivileged, older adults.

“You want to make sure that the people who are healthy and who have health problems are all getting in there, so the more people you have, the lower the chances that any one person will cost too much,” said David Barnes, policy director at Generation Opportunity in Arlington, Virginia. “A few really expensive cases average out over people who don’t have any problems.”

Millennials do approve of one aspect of the ACA, however: They like that they can stay on their parents’ insurance until they’re 26 years old. Other than that, the insurance plans provide extensive coverage they don’t need. Instead of allowing the insurance companies to figure out what plans would best appeal to their market, the government has mandated certain types of coverage.

Related: Obamacare Premiums Surging in Swing States

For instance, millennials are required to pay for a plan that covers rehabilitative services, such as speech therapy after a stroke or cardiac and pulmonary rehab. Most will never need this medical care. Nor will they likely need coverage for transplant surgeries or nursing home care — both required under Obamacare coverage.

Millennials are required to pay for a plan that covers rehabilitative services they’ll likely never use.

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“It’s an expensive insurance plan for people who would never need it. People in their 20s go to the doctor very rarely,” Barnes told LifeZette. “They go to one or two preventive visits a year, and spending a few thousand dollars for that is a lot.”

The cheapest plans on the Obamacare exchanges in most states cost close to $300 a month in premiums and have deductibles close to $5,000 or $6,000. Young adults don’t have that money to spend. Eight percent of them are unemployed — higher than any other demographic. They are also shouldering a total of $1 trillion in student loan debt — which averages to about $26,600 per student, or $38,600 with interest.

With all that debt, they better hope they studied science, engineering, or business; students who studied visual or performing arts have even higher unemployment rates than the rest. More than 37 percent of college graduates are working jobs that require nothing higher than a high-school education, according to the Bureau of Labor Statistics. Five million graduates are working jobs that require less than a high-school education.

[lz_bulleted_list title=”Millennials and Health Care” source=””]The cost of treating 18-to-24-year-olds averages $1,834 per person annually, compared to $2,739 for people ages 25 to 44 and $5,511 for those ages 45 to 64, according to the Kaiser Family Foundation.|Millennials 18 to 34 are most likely to ask for a discount, ask for a cheaper treatment option, request a price check or appeal an insurance decision, according to a report from PwC’s Health Research Institute.[/lz_bulleted_list]

President Obama is still trying to win over millennials, however. He began the Millennial Outreach and Engagement Summit in September, hoping the younger generation will salvage the failing health law. But this effort isn’t likely to work. Barnes suggested insurance companies be allowed to do market research to determine the most attractive plans for younger generations.

“Insurers can do what companies are supposed to do and find out what people want — and offer to sell those things,” said Barnes. The system won’t work until people begin “voting with their dollars” on the plans that offer the types of coverage they really need. But a generation strapped for cash isn’t going to voluntarily subsidize the health care of older, sicker generations without more incentives.

The plans now leverage large monthly costs against smaller deductibles — versus smaller monthly costs and huge deductibles. That’s the only form of competition in the Obamacare markets, and it’s stifling innovation. This kind of competition appeals only to those people who already desperately need coverage, not to the healthier demographics.

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“Insurance companies advertise to the healthy. They don’t advertise to the people who have problems,” said Barnes.

If millennials don’t start buying Obamacare insurance policies, the public marketplaces will sink under mounting costs.