But the reality for Mr. Biden is that while running mates are second-fiddlers by definition, the phenomenon of Ms. Palin has rendered him something of a fourth or fifth fiddle. It is not like last month, when reporters swarmed Mr. Biden’s Delaware home and delegates swooned at the Democratic convention. He is now trailed by just a few national reporters, and struggling to break through in a race marked by historic firsts, political celebrities and charismatic newcomers  none named Joe Biden.

The Obama campaign was hoping to reintroduce Mr. Biden this week as running mate attack dog. But his penchant for verbal rambling ensured that much of the attention he drew was unwanted: he said wealthy Americans had a “patriotic” duty to pay more taxes, a remark the McCain campaign mocked relentlessly.

Yet Joltin’ Joe has also become a fascinating Off Broadway spectacle in his own right. He is a distinctive blend of pit bull and odd duck whose weak filters make him capable of blurting out pretty much anything  “gaffes,” out-of-nowhere comments (pivoting midspeech to say “Excuse my back!” to people seated behind him), goofy asides (tapping a reporter’s chest and telling him, “You need to work on your pecs.”)

Mr. Biden’s role is red-meat serious: to pulverize Mr. McCain, lend foreign policy heft to Senator Barack Obama and be his campaign’s main ambassador to two at-risk constituencies: former supporters of Senator Hillary Rodham Clinton and blue-collar Democrats. He speaks to working-class voters in the harsh language of their economic trials, and summons easy rage at ear-splitting volumes.

“John said that the economy can have a ‘psychological effect,’ ” Mr. Biden said at a twilight rally in Media, Pa., on Tuesday. “Let me tell you about the psychological effect,” he said, describing his family’s economic struggles when he was young. Mr. Biden might seem like an afterthought on the national airwaves, but he can clearly resonate close up.