In what way has belief in the Trinity decreased?

I have heard that over half of Protestants do not believe in an earlier theological conception of the Trinity. I have heard that this is especially true of Evangelicals, and that they consider the older type of belief to be Catholic in nature. In what sense has personal belief in (this older Protestant conception of) the Trinity decreased? Why? Note that I am not asking whether Christians believe in the Trinity (most do), nor about Church doctrine, but about some sort of broad shift in belief. This poll provides an example, though I am not asking for analysis of that poll but rather about what further knowledge people have on this.