In my forthcoming book, I look at various trends of Evangelical Christianity over the decades. Perhaps the most basic trend to consider is how many Americans affiliate with an Evangelical denomination/ church. This question is answered with data from the General Social Survey, as shown in the figure below.
As you can see, the two big changes have been the significant decrease in number of Mainline Protestants and the increase in religiously unaffiliated. Evangelicals? They've held steady at 25% of the population for several decades now.
(Details: I measured affiliation using the RELTRAD coding scheme and then ran a smoothing algorithm for each group.)