THE DEMOCRATIC NATIONAL CONVENTION exposed the central defect of Senator Barack Obama’s candidacy: the absence of compelling evidence he is up to the job of president. The exposé comes courtesy of a bad habit of his running mate, Senator Joe Biden. When in doubt, Mr. Biden exaggerates. And in the past week, he did a lot.

Voters expect candidates to embellish, but only so much. Go beyond acceptable stretching and a candidate may squander his most precious political possession: credibility. Mr. Obama may be on this perilous path.