The outcome of a 100-year fight for U.S. national health care rests on the verdict today of nine justices, who will emerge from behind a red velvet curtain.

Just four months before the presidential election, the Supreme Court is poised to rule on the constitutionality of President Barack Obama's biggest legislative achievement, which would extend coverage to at least 30 million uninsured Americans in the biggest overhaul of the nation's health-care system since Medicare and Medicaid were enacted in 1965.

"When we talk about Obama 20 or 30 years from now, this is likely to be the bill we talk about," said Julian Zelizer, a presidential historian at Princeton University. "If the Supreme Court takes away from Obama his biggest accomplishment, this is exactly what a president really fears. In some ways, it's worse than not getting re-elected."