For a brief time in mid-October, some Democrats believed Michelle Nunn, the party's Senate candidate in Georgia, could be the firewall that prevents a Republican takeover of the Senate. If Nunn could win the seat opened by retiring GOP Sen. Saxby Chambliss, then Republicans would need to pick up seven, not six, seats to take control. In half-a-dozen polls taken over a two-week period in the middle of October, Nunn led Republican opponent David Perdue in five, while the candidates tied in one. Democratic optimism surged. "National Democrats have just decided to pour $1 million into this race in Georgia,...