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.

Shmuel Johnson, who works in Los Angeles at a small sound studio, hasn’t seen any ads or perused the state’s health exchange.

"There’s this elitist attitude that (politicians) think they know what’s better for us than ourselves and that’s part of why I take issue with this. I’m being forced to do something that’s not necessarily in my best interest," said Johnson, a 31-year-old who’s never had insurance. "I don’t need insurance, man. I’m healthy."

He’ll wait until March to enroll and says he’ll select the cheapest, lowest-level of coverage available simply to avoid the fine. Experts expect many young adults, like Johnson, to wait until March.

In 2012, 18 million 19 to 34-year-olds lacked insurance — or 27 percent of all people in that age group, according to U.S. census data.

The Obama administration is making the rounds on college campuses to encourage people to sign up and has enlisted celebrities including Lady Gaga and Kerry Washington in its Get Covered social media campaign. Jennifer Hudson and Olivia Wilde were featured in skits pushing the Affordable Care Act on the humor website FunnyorDie.com. In the latest push, an Obama impersonator encourages young adults to tell their friends to get covered in an online rap.

The president himself recently told a group of mothers visiting the Oval Office that: "Moms can tell young people who think they’re invincible that they’re not and prod them to at least get information."

California state exchange officials even tried to persuade women to pay the first month’s premium as a Christmas gift to their adult children and grandchildren.

Experts say engaging young invincibles requires a nuanced touch. They prefer to talk with their peers about pragmatic things they can do to impact the world, but aren’t interested in ideological debates, said Morley Winograd, author of 3 books on millenials, including "Millenial Momentum."

But the cost of coverage will play the biggest role, experts say.

More than 3 million young adults have health insurance thanks to the Affordable Care Act because they remained on their parents’ health insurance, according to the feds. The law extended the age that children can stay on their parents’ plan to 26.

Joshua Benson stayed on his parents’ insurance until he turned 26 last year. After that, Benson, who had his pancreas removed and needs daily insulin for his Type 1 diabetes, struggled to find coverage. He was either denied or quoted $2,000 monthly premiums, said the South Florida resident, who works part-time as a grocery store cashier.

He recently enrolled in a platinum plan with no deductible that costs him $170 a month and even covers his endocrinologist. The federal government kicks in another $200 a month.

Benson says he was amused by the Funnyordie.com skits, but said many other ads "are focusing more on getting our attention than actually giving us any valid information."

On the other side of the aisle, groups that oppose the health overhaul such as Generation Opportunity are spreading their message at college tailgate parties. The organization gained a following after disturbing-by-design social media videos featuring a creepy Uncle Sam popping up at gynecological and proctology exams went viral. The tagline urged young adults to keep big government out of their personal health decisions.

The group’s recent tailgate party at the University of Miami had all the markings of the South Beach club scene: hired glossy-haired models handing out swag, free alcohol and a sea of sweaty twenty-somethings bumping and grinding to a live DJ.

Mette Jensen, a 22-year-old student, says she supports "Obamacare" even though she signed a petition against it.

"Well, why not. I love free stuff."

Associated Press writers Brady McCombs in Salt Lake City, Utah and Gillian Flaccus from Los Angeles contributed to this report.