Joseph I. Lieberman, lapsed Democrat of Connecticut, strolled into the weekly lunch of the Senate Democrats last Tuesday, unaccompanied by a food taster.
He greeted his colleagues, including some who felt he should not have been there. He ate his lunch (salad, eschewing the mac and cheese) and sat through a discussion about gasoline prices and Medicare.
Then the conversation veered into the danger zone, the presidential election -- specifically, Senator John McCain's recent votes, or nonvotes, on energy policy.
At which point Mr. Lieberman walked out.
"I just didn't feel it was appropriate for me to be there," Mr. Lieberman explained the next day.
"It was the right thing to do," said Senator Richard J. Durbin of Illinois, the Democratic whip, who said that a colleague approached him afterward to complain about Mr. Lieberman's showing up. "This is a delicate situation," Mr. Durbin summed up.
It has grown increasingly so for Mr. Lieberman, once his party's vice-presidential candidate and now a self-styled "independent Democrat." He has zigzagged the country on behalf of Mr. McCain, the presumptive Republican presidential nominee, and, in recent weeks, amplified his criticism of Senator Barack Obama to a point that has infuriated many of his Democratic colleagues.