“I don’t understand why the left of the left has decided that this is their Waterloo,” said a senior White House adviser, who spoke on the condition of anonymity. “We’ve gotten to this point where health care on the left is determined by the breadth of the public option. I don’t understand how that has become the measure of whether what we achieve is health-care reform.”

“It’s a mystifying thing,” he added. “We’re forgetting why we are in this.”

Another top aide expressed chagrin that a single element in the president’s sprawling health-care initiative has become a litmus test for whether the administration is serious about the issue.

“It took on a life of its own,” he said.