US citizens should feel great about the fact there are three solid, capable people in the running to be their next president. The Pennsylvania primary results are interesting: Clinton's margin of victory, while less than polls suggested two months ago, was almost double what pundits were forecasting just before the voting. But over 80% of Pennsylvania voters said they felt the campaign had turned too negative, and blamed Hillary for that.
So why did she do so well? I think her own assessment in this case (I usually don't agree with her) is correct: People understand the US presidency may be the toughest job in the world. If any candidate can't take some punches, avoid getting defensive, hit back without lowering their own stature, and rise above the criticisms, people assume he or she will likely have a hard time dealing with the much heavier, much more consequential pressures in the White House.
When other Democratic leaders plead for less negativity, thinking they may be helping Obama stand up against Clinton's attacks, they actually may be hurting him. For the average voter, that may raise the question, "Why does he need to be protected?"
There is potential Hillary's attacks may backfire. While both Bill and Hillary can be about as pleasant and empathic with common people as we've ever seen in America, they've each shown vicious streaks at times that are pretty scary. If candidate Clinton goes too ad hominem in her attacks or sounds too shrill, then the focus will turn to "Is she resorting to personal attacks because she can't win on the merits of her own positions and character traits?"