Sen. Joseph Lieberman, speaking in that trademark sonorous baritone, utters a simple statement that translates into real trouble for Democratic leaders: "I'm going to be stubborn on this."

Stubborn, he means, in opposing any health-care overhaul that includes a "public option," or government-run health-insurance plan, as the current bill does. His opposition is strong enough that Mr. Lieberman says he won't vote to let a bill come to a final vote if a public option is included.

Probe for a catch or caveat in that opposition, and none is visible. Can he support a public option if states could opt out of the plan, as the current bill provides? "The answer is no," he says in an interview from his Senate office. "I feel very strongly about this." How about a trigger, a mechanism for including a public option along with a provision saying it won't be used unless private insurance plans aren't spreading coverage far and fast enough? No again.

So any version of a public option will compel Mr. Lieberman to vote against bringing a bill to a final vote? "Correct," he says.

This is, of course, more than just one senator objecting to one part of health legislation. This is the former Democratic vice presidential nominee, now an independent, Joe Lieberman, still counted on to be the 60th vote Democrats will need to force a final vote on health legislation. In opposing a public option, he is opposing the element some Democratic liberals have come to consider the cornerstone of a health-care bill.