Image caption Senate Republican Leader Mitch McConnell vows the healthcare law will be a "big issue" in the 2010 election

Virginia can challenge in court the constitutionality of President Barack Obama's signature healthcare reform law, a judge has ruled.

The decision is a setback to the administration, which will now have to defend the law in court in the run-up to the November elections.

The state argued the law's requirement that Americans purchase health insurance is unconstitutional.

The state's governor and attorney general are Republicans.

The Obama administration has argued the requirement is structured like a tax on those who do not purchase insurance and that the federal government has broad constitutional authority to design tax policy.

US District Judge Henry Hudson rejected the federal government's motion to dismiss Virginia's suit. He emphasised his ruling was merely an initial step and said the issue was ripe for court review.

Mr Obama's Secretary of Health and Human Services Kathleen Sibelius told reporters she remained confident the healthcare law was grounded in the constitution.

'A monstrosity'

The healthcare overhaul was the centrepiece of Mr Obama's 2008 campaign platform and a key element of the Democratic Party's domestic agenda.

It aims to extend health insurance to millions of Americans who lack it now, in part by requiring the mostly young, healthy Americans who currently forgo insurance to purchase it, thereby enlarging the pool and curbing costs for the whole country.

The legislation passed with no Republican support and opposition to it is expected to be a major part of the Republicans' campaign this summer and autumn.

"This healthcare bill is a monstrosity and will be a big issue in the fall," Senate Republican Leader Mitch McConnell told Reuters news agency after the court ruling.

"We would repeal it and replace it were we given enough votes to do that."