MIAMI -- The so-called young invincibles are so important to the success of the Affordable Care Act that supporters and detractors are spending millions to reach them with racy ads, social media campaigns and celebrity endorsements. The president is even (gasp) asking their mothers to help convince them to sign up for insurance.

The federal government and states running their own exchanges have launched marketing efforts for this crucial demographic of healthy young adults, but it's unclear if the messages are getting through.

Eric Fisher, a 28-year-old from Salt Lake City, said he still hasn't seen any of the social media campaigns -- one of which targets Utah residents with images of people snowboarding and rock climbing.

He tried to sign up online when the federal marketplace first launched but couldn't because of the long wait times and other website glitches. He said he'll try again at some point.

He added that the historic health care overhaul isn't a topic he and his friends spend much time talking about.

"It's not like a coffee table conversation," Fisher said.

According to a recent Harvard survey, many of Fisher's peers are undecided.

A poll by Harvard's Institute of Politics shows about 40 percent of people between the ages of 18 and 29 are on the fence about whether to sign up, with the rest split fairly evenly between those likely to enroll and those who probably won't.

The survey of 2,000 young adults was conducted from Oct. 30 to Nov. 11, after the first month of enrollment on the health care exchanges and when sign-up problems were at their peak.

Consisting of healthy college students and twenty-somethings, the so-called "young invincible" demographic is the holy grail of the Affordable Care Act. Insurers need their participation to offset the costs of covering older, sicker Americans. If enough young people decide not to buy insurance through state or federal marketplaces, it could throw off the market's equilibrium and cause insurance rates to rise dramatically the following year.

Federal officials haven't released detailed demographic information on who's enrolled so far, so it's not clear how many young people have signed up.

Ad campaigns in many states are courting undecided young adults.

In Colorado, a nonprofit group created a series of provocative "got insurance?" ads. One features a blonde standing next to a life-sized cut-out of celebrity heartthrob Ryan Gosling with the caption, "Hey girl, you're excited about easy access to birth control and I'm excited about getting to know you.

She got insurance." Another touting "Brosurance" encourages men doing a keg stand not to tap into their beer money to cover medical bills. When the exchange launched, models wearing nothing but underwear and "Get Covered" signs passed out fliers in downtown Denver.

Arizona and Utah ads targeting weekend warriors and other athletes note the risks of getting hurt without health insurance.