The U.S. Supreme Court on Monday opens three days of hearings on the federal health care law, formally known as the Affordable Care Act, that President Obama casts as one of his most important achievements in office, while his opponents blast it as “Obamacare.” Here’s a closer look at the three days of hearings the court will hold and what they mean.

The court is likely to issue a final ruling in June.

1. What is the court deciding?

A group of Republican-led states, including Florida and Virginia, and other groups have filed suit against the health care law, arguing it is unconstitutional because it requires every American to get health insurance. Opponents say the federal government does not have the power to impose such a requirement.

The states also question the way the law requires them to spend additional funds in Medicaid, the health insurance program for low-income adults.

The court will examine both issues.

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The mandate is at the center of the health care provision President Obama signed into law in 2010. It reflects a few different ideas shared by Democrats and many non-partisan health experts: people with health insurance may be more likely to get cancer screenings and other care that prevents more serious illnesses later; health benefits aside, some people will choose not to purchase insurance unless it is mandated; and without a mandate, some will seek insurance only after they get sick.

The law requires health insurance companies to cover people even if they already have illnesses, so the mandate attempts to prevent people from gaming the system and only buying insurance when they become ill.

Without the mandate, the underpinnings of the health care law that Obama enacted would be threatened. Insurance companies, for example, could decide to drastically increase costs for their general consumers if they will also be forced to cover people who get insurance only after they have illnesses. It would take away from the goal of making sure all Americans have health insurance.

2. What is the law currently doing?

The full effects of the law, which would include a fine for people who don’t get health insurance, don’t kick in until 2014. Eventually, a system of subsidies will be created to make insurance affordable to millions of Americans who currently don’t have it.

But some of the benefits have already started. For example, people 26 and under can remain on their parents’ health insurance plan, a key benefit with unemployment remaining high.

Insurance companies can no longer have lifetime limits for how much they spend on covering a person.

Blacks have been key beneficiaries of these provisions. According to the White House, more than 400,000 African-American young adults have gained insurance who might not otherwise have been covered because of the 26 and under requirement.

Most of these benefits are not being evaluated by the court, but they could be weakened or eliminated if the court strikes down the law. (See more below)

If the court strikes down the mandate, it would weaken a law which disproportionately aids African-Americans, who are more likely than whites not to have health insurance. According to estimates from the U.S. Census Bureau, more than eight million blacks currently don’t have health insurance, and more than seven million are likely to get coverage over the next few years if the law stays in place.

3. What does this mean for President Obama?

Politically, the court deciding to strike down parts of the health care law may have only a limited impact. Polls suggest about 40 percent of the country wants the law gone, while about 40 percent support it, and the rest of the country is undecided. That tracks the political divide overall in America, as Democrats like the law in part because it was an Obama initiative, Republicans oppose it on similar grounds.

So it’s not clear if Obama will lose any votes if the court strikes down the law. Some Democrats argue the public will accept the law if it is less of a legal question, but they’ve been making a similar case (the law will become more popular as people learn of its benefits) that has so far turned out not to be true. Republicans are likely to a Supreme Court decision striking down the law shows the president is incompetent, but it’s not clear that make a huge impact with voters either.

But health care reform is perhaps the signature legislative achievement of the president’s tenure. The court gutting the law would damage Obama’s legacy as president.

4. How likely is the court to strike down the law or parts of it?

That’s very unclear. The court generally is split between four liberals, four conservatives, and Justice Anthony Kennedy, an appointee of President Ronald Reagan who could be the swing vote as it he often is.

But the court could make a more limited ruling based on the Anti-Injunction Act, which essentially says a suit cannot be filed to strike down a tax until that tax has been imposed. (The mandate penalty for not having insurance does not go into effect in 2014). The court could use that provision to say no one has standing yet to file a lawsuit to strike down the mandate.

The court could choose to strike the mandate, but leave the rest of the law in place. Or strike it all or strike none of it.