Borger doesn't attribute Palin fatigue entirely to the shootings: "You might argue it's because of the debate surrounding the Tucson shootings -- specifically, Palin's tone-deaf response to the unfair charges that she was somehow responsible for a deranged shooter's state of mind. And that could well be part of it. But there's more: She's completely overstayed her welcome."
Borger's article is definitely worth reading, and what Erick Erickson, editor of redstate.com, is quoted as saying may be true:
And there's one more thing we can't forget: Voters actually like positive leaders who inspire them. Relentless negativity can work, to be sure. Yet in the end, Americans gravitate to those who make us feel good about who we are capable of becoming.
Successful candidacies and presidencies are not just about the foibles of the opposition. They're about our own ability -- as John McCain was fond of saying -- to become a part of something greater than ourselves. Palin, alas, is still stuck on the "me" part.