A 20-year trend of declining church membership in mainline denominations and rapid growth in evangelical churches is starting to level off, says the editor of the Yearbook of American and Canadian Churches.

"Mainline losses continue but are moderating. At the same time, the trend toward gains in some conservative churches is also moderating," said Constant Jacquet, who edits the yearbook for the National Council of Churches' research office.>Jacquet based his conclusions on 1986 statistics gathered for the 1988 yearbook, the most recent statistics available for the 220 church bodies that participate in the annual survey.> Officials in some mainline denominations rejoiced at the news, but they wouldn't go as far as saying that the conservative tide is in retreat.> "There are conservative elements in most denominations, so it would be difficult for me to say that new church members are any more liberal or conservative now than they were a few years ago," said Warren Hartman, a longtime researcher for the United Methodist Board of Discipleship.> But he said contributing factors may be the church-seekers' growing uninterest in a church's denominational ties, a stronger emphasis on evangelism in mainline churches, and the increasing bureaucracy of burgeoning evangelical denominations.> Baby boomers _ who make up a great number of new church members _ seem to be more interested in the quality of Christian education available for their children than in the church's affiliation with a particular denomination, he said.> United Methodists and the United Church of Christ are among the mainline denominations that have been losing members since 1965.> United Methodist membership has dropped from 11 million in 1965 to 9.2 million in 1986.> Membership in the United Church of Christ has dipped from 2.2 million to 1.7 million in the same period.> The evangelical denomination that probably has benefited most from the gains of the past two decades is the Assemblies of God, which has gone from 572,000 members in 1965 to 2.1 million in 1986.