Do you understand why a Protestant would sympathize with the view that different people believing different things is good, and a Catholic would not?
No. Nor do I believe you reached that conclusion from any solid data but rather your own view of what Protestants are tolerant of. I reject the conclusion.
It's fine to believe it if you want, but I can't agree with you given my experience in the Protestant church. I am telling you based on my experience the reason there was a change in both Protestant and catholics totals is directly attributable to their leaders (priests, ministers, pastors). If you followed the campaign throughout you know Catholic priests were finally speaking about what what was right and what was wrong to their congregations. There was even movement to have kerry ex-communicated. When a spiritual leader takes scripture and the judgements of the Church seriously, their congregations behavioral patterns and thoughts begin to alter more in accordance with scripture. Similiarly, at the same time the Catholic church was begin to crack down certain protestant congregations were accpeting homosexuals as ministers. This set off a fire storm, one still occuring, and it's resulted in Churches breaking off from traditional protestant denominations or folding into non denominational sectors.
Protestants are not sympathetic to "anything feels good, beleive whatever you want". Otherwise they wouldn't be taking extreme steps of distancing themselves from a leadership that has become corrupt. The reason Protestant totals fell is because people have left the Protestant churches in favor of churches that will abide faithfully to scripture and THOSE churches saw increases in the exit polling date.
Of those that remain you have three segments. People intent on destroying the Church with Liberal doctrine. people fighting to save the church. People that know no better, and expect their "leaders" to spiritually guide them. Only one third of these people would fit your assumption and they are not Christians no matter what they state. 2/3rd's either are holding firm in fundmentalist approach to scripture or desirous of doing God's will by misled by their leaders.
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.