THE QUESTION no longer is whether Democrats can win back the Senate, now controlled 51-49 by Republicans. The prospect of a Democratic takeover was always remote and has recently become more so. The question now is how much can President Bush, should he defeat John Kerry by 5 percentage points or more, help Republican Senate candidates, assuming there's a coattail effect. And that leads to the more important question of how many seats Republicans might pick up. As many as four or five, or none at all?The trend at the moment is favorable to Republicans, but nothing is guaranteed. Of...