The individual mandate is among Obamacare's most hated provisions. About two in three of Americans think the requirement to buy health insurance is a bad idea.

But recent enrollment data shows that the mandate is working. The exact type of people the requirement was meant to target — young, healthy adults who might forgo coverage were it not for a government fine — signed up in record numbers this year.

Having a decent number of young and health people in the insurance pool is integral to making costs affordable for everyone, which is exactly why the mandate exists in the first place. And architects of Obamacare's enrollment strategy say that talking about the mandate — something Obamacare supporters didn't really start doing until 2015 — has been core to making it work.

Obamacare supporters used to skirt talk of the mandate

The earliest polls made the facts clear: Americans did not like the idea of the government requiring the purchase of health insurance. The mandate was the very first part of the law challenged in court.

Obamacare supporters took all of this in consideration as they crafted early messages.

"The first year we were concerned it would be interpreted as a negative message, possibly turning people off," says Anne Filipic, who runs Enroll America, a national nonprofit focused on getting the uninsured signed up for the health law's insurance expansion.

But 2015 was different. Survey research had shown that, despite the mandate's unpopularity, reminding the uninsured of the fees they'd face for remaining uninsured was an excellent way to encourage them to buy coverage. The penalty rose from $95 in 2014 to $695 in 2016.

"The first year we were concerned it would be interpreted as a negative message"

So this year, the mandate was "core" to Enroll America's message. "It's in our outreach script, it's in our email messaging, it's integrated into the top line points we want to get across," Filipic says.

New data suggests the new message was successful. In 2015, people under 35 made up 35 percent of Healthcare.gov's open enrollment sign-ups. In 2014, the number stood at 33 percent. What's more: Healthcare.gov netted 980,000 new enrollees under 35 this year, a big increase over the 670,000 new sign ups last year.

The percentage point increase is, to be sure, slight. But it does show that, in 2015, there was something that worked to convince way more young adults to sign up for coverage — people who sat out the chance to do so in 2013 and again in 2014.

The individual mandate motivates young people to sign up — a lot

If there's anyone who understands how young adults think about the individual mandate, it's Mike Perry and Tresa Undem. They run the polling firm PerryUndem that, for the past two years, has done some of the most extensive work surveying the Obamacare-eligible population.

They've consistently found that the individual mandate tends to be a stronger motivator for young adults (for them, people between 18 and 29) than any other demographic.

"They're not like other groups," says Perry. "In focus groups, they don't talk about wanting preventive care, or the importance of covering their family. Young adults really talk about two things: accidents could screw me over, and I don't want to pay the fine."

Perry and Undem conducted a survey in 2014 of the people who signed up for coverage in Obamacare's first enrollment period. They found that young adults (18 to 29) were more likely to say they signed up for coverage because "I didn't want to pay the fine" than any other demographic.

Perry pointed to data from Massachusetts' health insurance expansion, which phased its mandate in over three years — and saw young adult enrollment go up as the penalties grew higher.

For the first six months of the Massachusetts health insurance expansion (from July to December 2007), there was no penalty for not carrying coverage, and the average age of those covered was 45.1. In January 2008, the state began to assess a $219 penalty to the uninsured — and the average age of enrollment dropped to 43.3. The full mandate penalty of $900 started in January 2009, and average age fell again, this time to 41.3.

"My own work tells me there has to be a connection," Perry says. "For young adults, they don't have as many motivations as other cohorts. So when the fine goes up, so does their interest in getting coverage."

Pro-Obamacare groups are trying to make the mandate work even better

Enroll America knew, from its survey data, that the mandate was a good way to convince young adults to buy health insurance. And they've used the past year to try and perfect the message, to increase the odds that someone they email or call would end up enrolling in coverage.

For example, Filipic said that her group wanted to understand what they should call the individual mandate. Should it be the mandate? Or a tax? A fine? A penalty?

Enroll America tested out the different words in different versions in the subject lines of their emails, seeing which ones recipients were more or less likely to open. They found that fine worked best — so they went with that.

Another thing Enroll has learned: people like to know the size of the fine, so the group tries to feature that number prominently, too.

Filipic and her team already think the mandate could play a bigger role in their messaging going forward. Right now, Enroll America has a calculator that lets potential enrollees see how much financial help they'd be eligible to receive if they signed up for coverage.

The group is experimenting with changing that calculator to also display the size of the fine the individual would pay if they didn't buy coverage.

"The calculator is consistently the most visited page on our site, so we're testing different ways to incorporate that information," she says. "We want to give consumers specific information, related to their own situation, rather than generalities."

Filipic doesn't know exactly what that will look like — it will take a bit more testing. But she does know that the mandate, as part of Enroll America's message, is here to stay.

"The increase in young people is very encouraging," she says. "The fine is going up, and we're three years into this now. So the repeated message, seeing friends and family get coverage, all those things are now starting to come together."