Fewer than one-third of young, uninsured Americans say they are leaning toward enrolling in a health-care plan under the new Obamacare exchanges, according to a new poll -- a number that, if it holds, would present huge problems for the new law.

In order to keep costs down, the Affordable Care Act relies on younger, healthier people signing up for coverage to offset the costs for older, sicker Americans.

But a Harvard University Institute of Politics poll shows just 29 percent of uninsured 18-to-29-year olds say they will definitely (13 percent) or likely (16 percent) enroll in the Obamacare exchanges. When the question describes the law as the "Affordable Care Act" rather than Obamacare, just 25 percent say the are leaning toward enrolling or will enroll.

Young people aren't signing up for Obamacare health exchanges, a new poll shows. Aaron Blake tells you why this is a holiday present for the Republican party. (The Washington Post)

About the same number say they're unlikely to or definitely won't sign up. Another four in 10 say it's a 50-50 proposition.

Not enrolling would subject these people to a penalty under the individual mandate, but in the law's first year, the penalty is relatively small.

The slow pace of enrollments among young people has already been cause for concern. The White House has estimated that it needs 40 percent of enrollees to be under 35 years old, but early numbers in states where data is available suggest that that number is closer to about 25 percent.

This is despite ad campaigns that have been geared toward signing young people up. The most notorious of these campaigns featured young men who appeared to be college-age participating in a keg stand.

More broadly, young people's opinions of the health-care law are pretty much on par with their older cohorts.

At least 56 percent disapprove of the health-care law, regardless of which label is used to describe it. And while half of young people expect their health-care costs to rise under Obamacare, only about one in 10 say the costs will decrease.

As for the care they will receive, more than twice as many say it will get worse under Obamacare (40-44 percent) as say it will get better (17-18 percent).

Updated at 11:25 a.m.

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