Since Protestants have typically voted more in favor of the Reps than Catholics, I would reject this idea.
The reality is that the pastors and ministers of these Churches have been liberalizing their doctrine. As a result some of the congregation no longer know right from wrong. Perhaps you've followed, as example, the allowance of homosexual pastors.
Of the remainder, many have left in favor of churches that adhere strictly to scripture. They've blended into the evangelical movement, non denominational churches, some into the Catholic Chruch etc... The reason Catholics increased is the same reason Protestants decreased. Their Preists took an active role. Only in the case of catholics, the Priests have begun to speak out against the liberal doctrines coming from those within. It is the same with Jews. The Orthodox among them have stayed true to doctrine and are stregthening their numbers. There was an increase in the Jewish vote and we can attribute, I believe, directly to the attention paid to being true to their faith.
I don't know which idea you are rejecting. My comment was strictly to the Kerry's statement during one of the debates, where he said that he would not impose his Catholic beliefs on anyone. This statement, idiotic as it is, must have a superficial appeal to the Protestants.
Please see my #25 where I tried to clarify this.