The Affordable Care Act's third year wasn't an easy one. The law faced two obstacles, a Supreme Court challenge and a presidential election, that threatened to dismantle it altogether.

What's more, Obamacare's architects didn't have much space in which to influence the outcomes. They could put their top lawyers on the legal challenge or try to run an excellent reelection campaign. But at the end of the day, they weren't the Supreme Court justices; they weren't the American voters. In its third year, the Affordable Care Act's fate rested squarely in the hands of others.

As the health-care law turns 3 today, that's no longer the case. Obamacare still faces a slew of obstacles but, unlike those in the past year, these are ones where the Obama administration and its allies have significant sway over whether the law succeeds or fails.

The next year will be a vexing one for the Obama administration, and one of the most important. This is when, after years of painstaking preparation, the Affordable Care Act is supposed to deliver on its main promise: expanding health coverage.

This is when we'll see whether a few million Americans gain insurance — or tens of millions do. Here are some of the bigger obstacles that the law will face in its fourth year.

1. Extending the scope of Medicaid expansion. The Medicaid expansion was meant to cover 17 million Americans over the next decade. But when the Supreme Court ruled that states could opt out of that provision in the health-care law, governors took up the option in droves. Fourteen states say they won't expand Medicaid, while three more are leaning in that direction.

To expand the reach of Obamacare, it's up to White House officials to woo those opponents to participate in the program. Their efforts have, in some cases, worked: Florida reversed its position on the Medicaid expansion in February to support covering 1.3 million Americans. Most recently, the administration has begun exploring the possibility of using Medicaid dollars to buy private insurance coverage, an approach that has interested about a half-dozen states.

Whether those states ultimately sign on could have the biggest impact on Obamacare's goal: ensuring that more Americans gain health-insurance coverage.

2. Building the health-insurance marketplaces. The health-insurance exchanges are Obamacare's backbone. These are the online marketplaces -- something like an Expedia for health coverage -- where Americans can shop for private insurance or Medicaid coverage.

Buying health insurance, though, is a lot more complex than purchasing a plane ticket. People will need to answer questions about where they live, how much they earn and who their family members are to determine what product they are eligible to purchase. To answer those questions, the exchange will need to have technology to connect to a federal data hub (which is still under construction).

Xerox has drawn up this flow chart of how an exchange works that underlines the complexity of the new market:

Initially, the Obama administration urged all states to build their own health-insurance exchanges. Only 17 states and the District of Columbia undertook the task. Seven states are assuming some of the responsibilities of the exchange, leaving 26 states in the federal government's hands.

The exchanges must launch for open enrollment on Oct. 1 of this year. That means that both the states and the federal government have just a few months left to finish building the technology and ensuring that it can connect to the national data hub and is easy for consumers to navigate.

3. Getting the word out about the the health law's new program. Last fall, the pro-Obamacare group Enroll America asked low- to middle-income Americans — precisely the people who would be eligible for new benefits — whether they were aware of the new law's provisions. Seventy-eight percent were not. Even when they learn about the health law, many remain skeptical that they will benefit from it.

The most popular provisions of the law have gone largely unnoticed, as the Kaiser Family Foundation found in its most recent tracking poll. At the same time, unpopular parts of the law, such as the individual mandate, are very well known.

The Obama administration will launch its big outreach campaign this summer, when the roll-out of the law's benefits is a bit closer. That's when it, and groups such as Enroll America, will be talking about the popular parts of the law — the features they expect Americans will really like. They have a big knowledge gap to close, and a short time in which to do it.

4. Swaying public opinion on Obamacare. Ever since the Affordable Care Act became law in 2010, public opinion has remained stubbornly split. The same number (40 percent) oppose it now as did three years ago. Favorable ratings, meanwhile, have fallen by 9 percent. 
Better polling numbers aren't necessarily essential to Obamacare in the same way that building insurance exchanges and expanding Medicaid are. Still, favorable opinions of the law would probably help sway Americans to sign up for its programs — or governors to expand Obamacare provisions in the first place. The White House could also have significant influence on how these numbers change, depending on how smoothly the law's big programs are implemented.

5. Controlling health-care costs. It's one thing to hand out health-insurance cards; that's relatively easy. It's quite another to ensure that an insurance card guarantees access to affordable health care. 

Right now, that's looking surprisingly possible: Health-care costs have grown at abnormally low rates for three years running. Some see this as a permanent trend in health-care spending, while others argue that it's a temporary side effect of the recession.

Federal forecasters do expect a big bump in health-care spending next year, when millions of Americans enroll in subsidized health plans. But going forward, a big question looms about whether the Affordable Care Act can do more than expand coverage — whether it can also control costs.

The law contains a number of experiments designed to drive down health costs, such as  Accountable Care Organizations. The whole idea is to move the system away from paying for volume and toward paying for value. We still don't know whether that will happen. But it's fair to say that reducing the cost of health care will make it easier to expand coverage.