In clinching the Democratic Presidential nomination, Barack Obama is now the leader of his party. The first test of his leadership as a potential President will be his response to the extraordinary campaign already underway to bully him into choosing Hillary Clinton as his running mate. Mrs. Clinton's tenacity in the primaries deserves credit, and she has earned the respect that Mr. Obama's remarks on Tuesday night gave her. But as he prepares for November, and perhaps for January, he has earned the right to choose a Vice Presidential nominee with whom he's comfortable, and on his own timetable. On...