The US Supreme Court has announced it will hear a constitutional challenge to President Barack Obama's healthcare reform law.

The court agreed on Monday to conduct a thorough review of the Obama administration's historic law with a ruling expected to be issued by July - just four months ahead of the 2012 presidential elections.

A group of Republican state governors and attorney generals brought the challenge to the Supreme Court. They argued that it was unconstitutional for the federal government to order individuals to purchase a healthcare policy or face a penalty.

The decision had been widely expected since September, when the Obama administration asked the country's highest court to uphold the centrepiece insurance provision and 26 of the 50 states separately asked that the entire law be struck down.

The law, passed by congress last year on a largely party-line vote, was the most far-reaching domestic legislation in a generation.

The law would extend health insurance to nearly 30 million Americans. It would prohibit insurance companies from charging higher premiums for people with pre-existing health problems.

'Significant timing'

Al Jazeera's Kimberly Halkett, reporting from Washington, DC, said, the question at the heart of the dispute is whether congress overstepped its power in requiring Americans to buy health insurance.

"Twenty-six states, more than half in the United States, say yes. They are now asking America's high court to strike down the healthcare law, set to take effect in 2014," she said.

"It's no surprise that the Supreme Court has decided to weigh in. What is significant is the timing. The justices are expected to issue their ruling in the midst of the 2012 election campaign.

A decision by July would take the healthcare debate to the heart of a presidential election campaign that ends with a vote on November 6 next year.

The law is vigorously opposed by all of Obama's prospective Republican opponents in next year's presidential election.

A ruling striking down the law while Obama seeks another four-year term would be a major blow for him legally and politically.

A ruling upholding the law would vindicate Obama legally, but might make healthcare an even bigger political issue for the leading Republican presidential candidates.

Polls show Americans are deeply divided over the overhaul, Obama's signature domestic achievement.