The figures come as the Supreme Court prepares to release its ruling on the Affordable Care Act, which remains unpopular with the public. A decision is expected within two weeks.
Families USA praised the law in the report and blasted "well-funded, right-wing activists [that] are intent on securing its demise."
"The new healthcare law can help stem the rising tide of the uninsured … If the law is struck down, the effects would be catastrophic," the report stated. "Without the law, the number of uninsured will continue to rise … And, worse still, many will die prematurely."
In 2014, the law's insurance exchanges will open and its Medicaid expansion will take effect, expanding coverage to millions. These developments will go by the wayside if the court strikes down the law.
Families USA Executive Director Ron Pollack called premature deaths among the uninsured "an American tragedy and an American shame."
"The Affordable Care Act lets us wake up from this terrible healthcare nightmare of premature death. Wiping out health reform means the nightmare will continue for all Americans," he said in a statement.
Critics of the law say its provisions are onerous and fiscally irresponsible.