WASHINGTON (AP) ― The monumental fight over a health care law that touches all Americans and divides them sharply comes before the U.S. Supreme Court on Monday. The justices will decide whether to kill or keep the largest expansion in the nation’s social safety net in more than four decades.

Two years and three days after President Barack Obama signed into law a health care overhaul aimed at extending medical insurance to more than 30 million Americans, the high court begins three days of hearings over the law’s validity.

The challenge from 26 states and a small business group puts the court smack in the middle of a heavily partisan fight over the president’s major domestic accomplishment and a presidential election campaign in which all his Republican challengers oppose the law.

If upheld, the law will force dramatic changes in the way insurance companies do business, including forbidding them from denying coverage due to pre-existing medical conditions and limiting how much they can charge older people.

The law envisions that insurers will be able to accommodate older and sicker people without facing financial ruin because of its most disputed element, the requirement that Americans have health insurance or pay a penalty.

Another major piece of the law is an expansion of the government-funded Medicaid program for low-income Americans that will provide coverage to more than 15 million people who currently earn too much to qualify.

By 2019, about 95 percent of the country will have health insurance if the law is allowed to take full effect, the Congressional Budget Office estimates. About 50 million Americans currently lack health insurance coverage.

Reams of court filings attest that the changes are being counted on by people with chronic diseases, touted by women who have been denied coverage for their pregnancies, and backed by Americans over 50 but not yet old enough to qualify for Medicare, who face age-inflated insurance premiums until they become eligible for the government’s health insurance program for seniors at age 65.

Republicans are leading the fight to kill the law either by the court or through congressional repeal. They say the worst fears about what they derisively call “Obamacare’’ already have come to pass in the form of higher costs and regulations, claims that the law’s supporters dispute. All the Republican presidential candidates promise to repeal the law if elected.

“Obamacare has already proven unpopular and unaffordable,” House Speaker John Boehner said on the law’s second anniversary last week.

Polls have consistently shown the public is at best ambivalent about the benefits of the health care law, and that a majority of Americans believe the mandatory insurance requirement is unconstitutional.

The administration’s public education campaign has come under strong criticism from its allies who say the White House has been timid in the face of relentless Republican attacks.

Washington lawyer Walter Dellinger, who served in the Clinton administration Justice Department, said opponents have succeeded in keeping the focus on the insurance requirement instead of two provisions that will keep insurers from discriminating against sicker and older people. “The other two are very popular, and no one discusses them,” Dellinger said.

The White House has belatedly begun touting parts of the law already in effect, including allowing children to stay on their parents’ insurance plan until age 26 and reducing older Americans’ prescription drug costs.

Having rarely talked about the law since he signed it, Obama issued a brief statement Friday. “The law has made a difference for millions of Americans, and over time, it will help give even more working and middle-class families the security they deserve.”

The main event before the court is Tuesday’s argument over the constitutionality of the individual insurance requirement. The states and the National Federation of Independent Business say Congress lacked authority under the Constitution for its unprecedented step of forcing Americans to buy insurance whether they want it or not.

The administration argues Congress has ample authority to do what it did. If its action was rare, it is only because Congress was dealing with a problem that has stymied Democratic and Republican administrations for many decades: How to get adequate health care to as many people as possible, and at a reasonable cost.

The justices also will take up whether the rest of the law can remain in place if the insurance mandate falls and, separately, whether Congress lacked the power to expand the Medicaid program.

The court also will consider whether the challenge is premature under a 19th century tax law because the insurance requirement doesn’t kick in until 2014 and people who remain uninsured wouldn’t have to pay a penalty until they file their 2014 income taxes in early 2015.

Taking this way out of the case would relieve the justices of rendering a decision in political high season, just months before the presidential election.

The justices like to say they give the same attention to the small cases as the big ones. But everything about the court’s handling of health care suggests there is no doubt among the court’s six men and three women about the significance of what they are about to decide.

The six hours of argument time is the most scheduled since the mid-1960s. The court will release audio recordings of the arguments on the same day they take place. The first time that happened was when the court heard argument in the Bush v. Gore case that settled the 2000 presidential election. The last occasion was the argument in the Citizens United case that wound up freeing businesses from longstanding limits on political spending.

Outside groups filed a record 136 briefs dealing with the four issues the court will take up over the next three days.

Justice Clarence Thomas, remarking on the sheer volume of the health care filings, noted they filled a large mail bin. “I said, `Oh my goodness, look at all that work,’” Thomas told law students at Wake Forest University. The school posted a video of his talk.

The case arrives at a high court in which ideology and political affiliation align for the first time in generations. The four Democratic appointees make up the liberal wing, while the five justices named by Republican presidents form a cohesive conservative majority on several key issues.

The partisan battle lines were drawn early on. The law passed Congress, controlled by Democrats in 2009 and 2010, without a single Republican vote.

Republican elected officials filed suit in federal court the very day Obama signed the bill into law.

Even in the courts, the first decisions fell along party lines. Democratic-appointed judges uniformly upheld the law or dismissed suits against it, while Republican appointees in Florida and Virginia struck it down.