The dems did that as a political manuever.
It's an election year and they don't want to appear weak on terror. Think of the field day McCain would have if Obama HAD voted against the wiretapping. It would be a pretty big weapon in the Republican mudslinging arsenal.
I agree that was their thought process, but I think they were wrong for three reasons:
#1, I don't believe that it would have hurt Democrats. It's not a political negative to stand up to the Bush administration.
#2, I think it actually hurts Democrats when they do this, because they try to look "strong" on national security by stating principles and then abandoning them to the Republicans out of fear that they'll appear "weak." When really, I'd assert, they look "weak" by giving in to all of President Bush's demands.
#3, I think it's stupid (some might say, "weak") to try to govern on the basis of how your political opponents will attack you. What's more, their opponents are Republicans -- they're going to attack them for being "weak" whatever they do. It doesn't matter. Get ahead of the issue and stand for something (unless, as I'm sure is the case with some of the Democrats, they just want more government surveillance power).
Cleo