The US Supreme Court on Thursday upheld the heart of President Barack Obama's healthcare overhaul, handing him a huge election-year political victory and keeping in force the legislative centerpiece of his term, a law aimed at covering more than 30 million uninsured Americans.

The decision means the historic overhaul - opposed by virtually all Republicans including Mitt Romney, Obama's challenger - will continue to go into effect over the next several years, affecting the way that countless Americans receive and pay for their personal medical care.

The decision, written by conservative Chief Justice John Roberts, sided with Obama on the centerpiece of the law, which requires all Americans to have health insurance or face a federal fine.