A study by Charles Blahous, a George Mason University research fellow and the Republican trustee for Medicare and Social Security entitlement programs for the elderly, challenges the administration's contention that the 2010 law would better keep healthcare costs in line.

Known as the "Affordable Care Act" or "Obamacare," the measure to expand health insurance for millions of Americans is considered Obama's signature domestic policy achievement.

The Supreme Court is currently weighing whether Congress overstepped its authority to regulate commerce in approving the law. The justices heard arguments in the high-stakes case two weeks ago.

Republican presidential candidates have promised to repeal it if one of them wins the White House. Conservatives denounce the standard as an unwarranted government intrusion.

Obama and Democrats believe the law will control skyrocketing costs and curtail government "red ink."

But Blahous, a former economic adviser in the George W. Bush White House, said in his research that the law is expected to boost net federal spending by more than $1.15 trillion and add between $340 billion and $530 billion to deficits between 2012-21.

"Relative to previous law, the (healthcare law) both exacerbates projected federal deficits and increases an already unsustainable federal commitment to health care spending," he concluded.

The analysis, first reported by the Washington Post late on Monday, also comes a month after the Congressional Budget Office (CBO) cut the estimated net cost of the healthcare law by $48 billion to $1.08 trillion through 2021.

(Reporting by John Crawley; Editing by Lisa Shumaker)

• A Permanent Economic Emergency