“I don’t have any line that I have in my mind,” Mr. Lieberman said in an interview. “If it happened, I’d know it when I saw it.”

Image Senators Joe Lieberman and John McCain after a meeting with French President Nicolas Sarkozy in Paris earlier this year. Credit... Philippe Wojazer/Reuters

Mr. Lieberman was leaning back in a chair in his Senate office, wearing a loose-fitting pinstriped suit, grinning a lot and appearing quite comfortable while describing “my uncomfortable position.” He compared his predicament to the old Groucho Marx conceit, “I don’t care to belong to any club that will accept me as a member.”

As Mr. Lieberman spoke, a group of protesters not far from his office were calling for his party to oust him as chairman of the Homeland Security and Governmental Affairs Committee. The organization, unsubtly named LiebermanMustGo, was delivering a petition proclaiming the same.

Will the protest or petition have any effect on him? “No,” he said, shrugging, which is his preferred coping method (and which he did about two dozen times in 45 minutes).

“I think most people in his caucus expected Joe’s views on national security, but I think the extent of his embrace of McCain has surprised some people,” said Senator Susan Collins, Republican of Maine, one of Mr. Lieberman’s closest friends in the Senate. “That’s taking an extra step.”

Democrats complain that he has gone even further with his ramped-up attacks on Mr. Obama. “The fact that the spokesperson for Hamas would say they would welcome the election of Senator Obama really does raise the question, Why?” Mr. Lieberman said recently on CNN. A few days later on Fox News he called Mr. Obama “naïve” in his views on Iran.

In his office on Wednesday, Mr. Lieberman spoke of what he called Mr. Obama’s “remarkable change of position” on a variety of issues.