WASHINGTON  Just the thought of Joseph I. Lieberman makes some Democrats want to spit nails these days. But Mr. Lieberman, the Connecticut independent, is not the least troubled by his status as Capitol Hill’s master infuriator  and on Monday he showed how powerful that role can be at a time when Democrats cannot spare a single vote.

The day before, Mr. Lieberman threatened on national television to join the Republicans in blocking the health care bill, President Obama’s chief domestic initiative. Within hours, he was in a meeting at the Capitol with top White House officials.

And on Monday night, Democratic senators emerged from a tense 90-minute closed-door session and suggested that they were on the verge of bowing to Mr. Lieberman’s main demands: that they scrap a plan to let people buy into Medicare beginning at age 55, and scotch even a fallback version of a new government-run health insurance plan, or public option.

Mr. Lieberman said he believed that the Medicare expansion was off the table, though he did not get any guarantee. “Not an explicit assurance, no,” he said. “But put me down tonight as encouraged at the direction in which these discussions are going.”