Democrats are keenly aware of the risks ahead, which is why it has been so difficult for House Speaker Nancy Pelosi (D-Calif.) to round up the votes. Many Democrats, recalling the debacle after their failure on health care in 1994, think that another failure will be equally costly. Others say there will be a price to be paid no matter what happens.

“The political consequences of 1994 took a full decade for the D Party to undo and reverse,” said William Galston, a senior fellow at the Brookings Institution and a domestic policy adviser in the Clinton White House. “If the political consequences of this effort turn out to be as long-lasting as 1994, that would be a very significant price that will have to be weighed in the historical balance.”

But will Republicans regret their unanimous opposition? Historian Kennedy sees dangers for the GOP if a reformed health- care system turns out to be as popular as Social Security and Medicare.

“They confer real benefits on people that are palpable, and people believe in them,” he said of those two programs. “What the political calculus is [among Republicans] that lets this come through a strictly Democratic proposal is a pretty high-stakes gamble.”