WASHINGTON » As a roller-coaster sign-up season winds down, President Barack Obama’s health care law has indeed managed to change the country.

Americans are unlikely to go back to a time when people with medical problems could be denied coverage.

But Obama’s overhaul needs major work of its own if it is to go down in history as a legacy achievement like Medicare or Social Security.

Major elements of the Affordable Care Act face an uncertain future:

— As a 6-month-long sign-up season comes to an end Monday the administration’s next big challenge is to make 2015 open enrollment more manageable for consumers unaccustomed to dealing with insurance jargon. There’s also concern premiums will rise next year.

— The new insurance markets created by the law are anything but customer friendly. After the HealthCare.gov website finally got fixed, more than 6 million people have managed to sign up, allowing the exchanges to stay afloat economically. But many consumers have bought policies with restricted access to top-tier hospitals and the latest medications. The website is seeing heavy traffic this weekend, and consumers may encounter a wait or last-minute glitches.

— Nearly half the states are still opposed to or undecided about the law’s expansion of Medicaid, the government’s health insurance program for the poor. As a result, millions of low-income people who otherwise would have been covered remain uninsured.

— This year’s pitch has been about the "carrots" in the law: subsidies and guaranteed coverage. But the "sticks" are just over the horizon: collecting penalties from individuals who remain uninsured and enforcing requirements that medium- to large-sized employers provide affordable coverage.

Many basic facts about the ultimate effects of the health insurance program remain unclear. It’s not known how many of those who have gotten coverage were previously uninsured — the ultimate test of the law. Independent measurements by Gallup do show fewer uninsured Americans, but such progress hasn’t won hearts and minds. The public remains deeply divided, with opponents of the law outnumbering supporters.

At a recent insurance industry conference, a top administration official acknowledged the huge job still ahead.

"The No. 1 thing that probably we’ve all learned from 2014 is that this is hard work," said Gary Cohen, outgoing director of the Center for Consumer Information and Insurance Oversight, the agency created to carry out the health care law. "It’s not a one-year project; it’s a multiyear project … we’re asking a lot, frankly, of consumers," he added. "This is new for them."

Among those consumers is Dan Luke of St. Paul, Minn., the owner of a small video production company who had been uninsured since he was turned down for coverage last year due to a pre-existing condition. The condition? Luke was born with one eye due to a birth defect, and he uses a glass eye.

"For 63 years I’ve had one eye," said Luke. "They had to dig deep to find that."

He’s happy with the coverage he and his wife have bought; they’re saving $300 a month on premiums compared with the last time they had insurance. But he said he had to endure weeks of website run-arounds.

"There is a lot of bureaucracy involved," said Luke. "It’s sort of like taxes, filled with loopholes and pitfalls. They should make it easier for people to get insurance and pay for insurance, rather than have to prove so many things and jump through so many hoops."

Those comments echo sentiments broadly reflected in national opinion polls. Most Americans want lawmakers to fix the problems with the health care law, rather than scrapping it. A new AP-Gfk poll finds that only 13 percent expect the law will be completely repealed. Seventy-two percent say it will be implemented with changes, whether major or minor.

Republicans have again made repeal of "Obamacare" their official battle cry this election season. But even if the GOP wins control of the Senate and Congress were to repeal the law next year, the president would veto it. Opponents would then need a difficult two-thirds majority in both chambers to override Obama’s veto.

"It’s going to depend on the next couple of elections whether we stick with the current ACA models," said Brookings Institution health policy expert Mark McClellan, who oversaw the rollout of the last major federal coverage expansion, the Medicare prescription drug benefit.

"We are still a long way from a stable market and from completing implementation," he said. But "we’re not going back to people with pre-existing conditions having no good options."

The administration will have to get to work quickly on a plan for next year. It is still struggling with such basics as providing consumers with clear information about the process and their options.

Until now, those signing up have skewed toward an older crowd. That could lead to higher premiums next year, making the program a harder sell for younger people.

Some Democratic lawmakers who voted for the law are frustrated.

"Instead of just circling the wagons against all the political arrows that are shot against this plan, we need a little more accountability, and we need to ensure the next enrollment period is not handled as poorly as the last one," said Rep. Lloyd Doggett, D-Texas.

DeAnn Friedholm, health reform team leader for Consumers Union, said her group still supports Obama’s overhaul, but with concerns.

"The jury is out in terms of its long-term success," she said. "We still think it’s better than the old way, which left a lot of people out because they were sick."