The phrase "mainline group" refers to a primary or established group or organization that forms the central or mainstream part of a particular category or movement. Full definition
Originally the study included only traditional mainline groups, but the participating denominations agreed to add the Southern Baptists to the study because of that denomination's thorough educational programs. (religion-online.org)
Contrary to other mainline groups, the UMC's stance is increasingly unlikely to change. (christianitytoday.com)
In working to maintain this favorable relationship with the networks, however, the mainline groups were under continual pressure. (religion-online.org)