The US Supreme Court's conservative justices pointedly challenged the core provision of President Barack Obama's historic healthcare law, casting constitutional doubt on the measure's key requirement that all Americans must buy insurance coverage or face a penalty.

The court's ruling is expected in June, in the middle of the presidential re-election campaign, and will touch the lives of virtually all Americans. Whichever way it decides, the court's ruling stands to deepen the ideological rift that has increasingly gripped the country during the Obama presidency.

The Obama plan would extend medical insurance to 30 million Americans who go without coverage, either by choice or because they can't pay the fast-rising premiums in the private insurance marketplace. Obama signed the measure into law two years ago, and it has since been challenged on constitutional grounds by 26 US states and a business organization. Opponents contend the law unconstitutionally extends the power of the federal government.

Before the reforms were signed into law, the United States was the only developed country without a national healthcare program.

With the heart of the Obama administration's healthcare overhaul hanging in the balance, the Supreme Court is turning on Wednesday to whether the rest of the law can survive if the crucial individual insurance requirement is struck down by the high court.

The justices also were scheduled to spend part of Wednesday, the last of three days of arguments over the health law, considering a challenge by 26 states to the expansion of the federal-state Medicaid program for low-income and disabled Americans, an important feature for the law's overall goal of extending health insurance to millions of Americans.

Associated Press