"Michael Dukakis spent the last months of the 1988 campaign calling his opponent's running mate, Dan Quayle, a risky choice and even ran a TV ad blasting Mr. Quayle. The Bush/Quayle ticket carried 40 states.
Adlai Stevenson spent the fall of 1952 bashing Dwight Eisenhower's running mate, Richard Nixon, calling him "the kind of politician who would cut down a redwood tree, and then mount the stump and make a speech for conservation." The Republican ticket carried 39 of 48 states.
If Mr. Obama keeps attacking Mrs. Palin, he could suffer the fate of his Democratic predecessors. These assaults highlight his own tissue-thin résumé, waste precious time better spent reassuring voters he is up for the job, and diminish him -- not her."
It's an interesting point. McCain hasn't actually been attacked recently, and Obama has begun to compare himself to Palin. That alone knocks him down a tier from where he needs to be.