Washington, Jun 29 (IANS): President Barack Obama scored a major victory in the run-up to the November presidential poll with the US Supreme Court in a historic verdict largely upholding his signature health care law.

Obama's stunning victory in a 5-4 split verdict Thursday was all the more sweet as the majority decision upholding the core of the law, dubbed "Obamacare", was written by conservative chief Justice John Roberts appointed by Obama's Republican predecessor George Bush,

With Roberts casting the fifth decisive vote, the conservative-dominated court essentially ruled that the so-called individual mandate requiring all people to have health insurance beginning 2014 or pay a fine was unconstitutional under the commerce clause, but would stand if seen as a tax.

"The federal government does not have the power to order people to buy health insurance. Section 5000A would therefore be unconstitutional if read as a command. The federal government does have the power to impose a tax on those without health insurance," Roberts wrote.

Four conservative justices disagreed. "To say that the Individual Mandate merely imposes a tax is not to interpret the statute but to rewrite it," they wrote in their dissenting opinion.

Careful not to spike the ball with the win on the politically divisive issue, Obama in a measured statement termed the apex court decision as a victory for all American people. Speaking from the White House shortly after the verdict, he said: "Whatever the politics, today's decision was a victory for people all over this country whose lives are more secure because of this law."

However, Obama's conservative opponents hoped to turn the stunning setback from the Supreme Court as a rallying cry for a turn to the court of public opinion in November to overturn the Affordable Care Act passed by Democrats in 2010.

Obama's likely Republican opponent Mitt Romney standing in front of the Capitol vowed to repeal the law on the very first day as his first act if he made it to the Oval office.

"Obamacare was bad policy yesterday, it's bad policy today. Obamacare was bad law yesterday, it's bad law today," he said. "If we want to get rid of Obamacare, we're going to have to replace President Obama. My mission is to make sure we do exactly that."

Twenty-six states, led by Florida, had challenged the law saying individuals cannot be forced to have insurance, a "product" they may neither want nor need.

Supporters argued the individual mandate is critical to the success of the legislation, because it expands the pool of people paying for insurance to make it affordable for all.