Vice President Dick Cheney's assertion that the nation was more likely to "get hit again" by terrorists if John Kerry was elected was one of the toughest attacks launched in a presidential election in 40 years. But Mr. Cheney's latest assault on Mr. Kerry, which startled Democrats and Republicans alike, raised a central question even in this notably ferocious presidential campaign: Is it possible for a candidate to go too far, and alienate the very voters he is trying to court?