In considering a Kerry-Bush run, the media is comparing Kerry to John Fitzgerald Kennedy. They were both Senators from Massachusetts. They were both war heroes. They were both rich.
There are a lot of reasons that Senators or Congressman are rarely elected President. The voting record is the biggest thing. Years and years of contradictory positions and speeches just make these folks fodder for any opponent. Governors generally don't have the same kind of paper trail. The time that they spent in Washington is also sure to have made a Senator or Congressman many enemies, from both sides of the aisle and from all regions of the country. Not so with Governors.
Kerry is different from John Fitzgerald Kennedy for a variety of reasons. First, Kerry has been in the Senate for a long time. Kennedy (J) was in the Senate for less than ten years before he was elected President. Kennedy played both sides of the fence from the very beginning. He was friendly with Joe McCarthy, even voting against his censure. Not so with Kerry. He has been a far-left liberal since the beginning of his Senate career. Only in the last few years has he even suggested that he is trying to move to the middle, and that went out the window when he had to move to the left to win the Democratic primary.
Kerry has a LONG record, and a record that is in many ways inconsistent with the positions that he is taking now. Is he entitled to change his mind? Yes. But he is not entitled to change his position in the middle of a presidential campaign without being made to explain WHY he has changed his mind. His many years in the senate will require quite a bit of explaining. That explaining will be a continuing distraction to his campaign.