It is a single Church only to the extent that its adherents are in communion with it. You can no more declare yourself a Catholic than you can declare yourself an NFL quarterback. As Socrates said: "To do is to be." If you claim that every church shopping and hopping Protestant is somehow intrinsically different than every lapsed and ex-Catholic you are going to have to substantiate that more thoroughly than simply pointing me to a website stuffed with irrelevant polling data.
“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. - Matthew 7:21
This is a purity argument, a theological argument, it has nothing to do with gathering voting numbers for Catholic voters, or Jews, or Protestants, or Muslims, or Mormons, etc.
The day the Vatican starts purging it’s rolls and offering a different set of numbers for how many Catholics are on the planet, then you can request pollsters to ask the Catholic voter if he is no longer Catholic, or has been told by the Vatican that he has been removed from being counted as Catholic, then the pollster can not count him as Catholic.