Her office puts out a good statement that reaches the correct conclusion:

Congress must vigorously check and balance the president even in the face of dangerous enemies and at a time of war. That is what sets us apart. And that is what is vital to ensuring that any tool designed to protect us is used – and used within the law – for that purpose and that purpose alone. I believe my responsibility requires that I vote against this compromise, and I will continue to pursue reforms that will improve our ability to collect intelligence in our efforts to combat terror and to oversee that authority in Congress.

I don't believe that if Clinton and Obama swapped roles that they'd be acting any differently. But the reality is that as long as Obama thinks he's going to be wielding executive authority, he's going to be useless as a check on out-of-control executive authority. If Clinton wants to channel whatever regrets she has about losing the primary into taking up that cause, well, I think that would be an excellent decision for her to make.