The implication of chp 7 in Andrew Greeley and Michael Hout’s book, The Truth about Conservative Christians, has an edge of irony or even humor — but statistically accurate: if the Mainline Christian couples want to have more influence in the elections, they need to leave their boycotts and city hall protests and get home, have child-producing you-know-what and start having more babies. Yep, you read it right.
What explanations are you hearing of why Mainliners are declining and Conservative Protestants growing?

One of the most interesting conclusions that Greeley and Hout discovered is that church growth begins at home, not with the evangelistic tract or evangelistic efforts in CP churches. Conservative churches are growing, not so much by attracting spiritually unfed liberals and mainliners, but by having women who statistically are more fertile. Conservative Christians have more babies than Mainline Christians. CPs have 2.1 children per family; Mainliners have 1.9 children per family. (Contraception is a major issue here.)
Excessive liberalism as an explanatory device for growth in CPs is a myth (in their opinion).
CPs [Cons Prots] have statistically grown for decades … not by evangelism so much as by having more children; and Mainliners are not holding their own when it comes to the statistical average (or they barely are).
Conversion stats — from Mainline to CPs — have remained steady for 75 years. 70% of the growth of CPs over Mainliners comes from fertility; the other 30% comes from a drop in conversions to Mainline churches.
Here are the possiblities and their statistical conclusions:
1. Increased conversion from Mainline to CPs? No.
2. Natural increase due to fertility? Yes (70-75%)
3. Decline in conversions from CPs to Mainline? Yes (25-30%)
4. More apostasy among Mainliners? No.
5. Greater inflow from outside Prots to CPs? No.
Significant: CPs have retained the upwardly mobile CP; in former decades many of the upwardly mobile converted to the Mainline churches.
The evidence is that this numerical growth advantage is about to level off so that the stats will remain about the same for the time being.