From the Pew Forum on Religion and Public Life:
The percentage of U.S. Catholics who consider themselves “strong” members of the Roman Catholic Church has never been lower than it was in 2012, according to a Pew Research Center analysis of new data from the General Social Survey (GSS). About a quarter (27%) of American Catholics called themselves “strong” Catholics last year, down more than 15 points since the mid-1980s and among the lowest levels seen in the 38 years since strength of religious identity was first measured in the GSS, a long-running national survey carried out by the independent research organization NORC at the University of Chicago.
The decline among U.S. Catholics is even starker when they are compared with Protestants, whose strength of religious identification has been rising in recent years. About half (54%) of American Protestants – double the Catholic share (27%) – described their particular religious identity as strong last year, among the highest levels since the GSS began asking the question in 1974.
You can examine this study in much greater detail here.