This isn't a matter of Catholic or Protestant, but a general trend that moderate, mainstream churches have seen a decline over the last few decades, while conservative congregations (such as Evangelicals) have remained relatively steady (not growing, but not losing members).
50 years ago, 99% of America was a member of a Christian church. Today, that number is 70% or less (depending on which survey you read). The 30% reduction comes entirely from those mainstream, moderate churches. The decline is the direct result of scandals (like the clergy pedophiles) and a general shift in the population's attitude toward religion in general.
Edit: The same day this question was posted, Five Thirty Eight posted a study that shows how liberals are fleeing religion, largely because of how the GOP and the Christian Right have formed an alliance over the last few decades.