The Senate chamber filled with audible gasps Tuesday when Sen. Edward M. Kennedy, the pro-choice champion, clearly voted ''yes'' on final passage of the bill to ban partial-birth abortion. It was a mistake, however, and Kennedy changed his vote a few minutes later. This marked the second time in a week that Kennedy, widely considered the most powerful member of the Senate, became confused on an important vote. He mistakenly voted with President Bush in opposing a Democratic-backed amendment to require partial Iraqi repayment of U.S. reconstruction aid. On that occasion, Senate Democratic Leader Thomas Daschle conferred with Kennedy and...