WASHINGTON  — The monumental fight over a health care law that touches all Americans and divides them sharply went before the 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 insurance or pay a penalty.