I don't disagree that the Catholic leadership in spots (not en masse) standing up for the Church was a factor in the phenomenon we discuss. The rash of gay "marriage" legislation was a factor energizing the religious right of all denominations, but it cannot be a factor among the mainline Protestants because the movement to mainstream homosexuality was doing very well in the mainline Protestantism.
We need to agree on the terminology. The article mentions decline in GOP support among mainline Protestants and increase among the fundamentalist Protestants. You correctly state that it is because there has been a movement away from the Protestant mainline.
This does not invalidate the fact that the Protestant mainline is inherently liberal because of the decentralizing tendencies in Protestantism since the reformation. You can dismiss them as "not Christian", but the article is not written from that sectarian assumption. They are just the kind of Christians you and I disagree with.
Kerry's statement on the inacceptability of Christian beliefs in politics is abhorrent tio Catholics, and it may be equally abhorrent to fundamentalist Protestants, but in the group where support for the GOP declined -- mainline Protestants,-- it is very much the conventional wisdom.
I think we're coming closer to agreement, though I'd stipulate this liberalizing of doctrine is not actually "mainline". Yet.
Certainly among the leadership it is becoming so, but if it were among congregants their numbers wouldn't be descreasing at the pace they are. Of course at the end once all those that take scripture seriously conclude their exodus, the majority of those remaining will fit your stated opinion and, infact, be the new "mainline".