In July of 2006, The Cook Political Report rated just 14 GOP-held seats as highly vulnerable. By November, the number of GOP-held seats in danger had tripled to 43. We saw a similar pattern in 2010. In August of that year, we listed 36 Democratic-held seats as highly vulnerable. By November, the number of vulnerable Democratic-held seats had more than doubled to 78. On Election Day of 2006, Republicans lost 30 seats; Democrats lost 63 seats in 2010.
So, what happened between the summer and November of those years that caused such a dramatic shift in ratings? CONT.
Amy Walter, Cook Political Report