Mainline churches is a term used to describe the main traditional Protestant denominations in the U.S. as differentiated from and on the theological left of evangelicalism. These denominations are viewed as having adopted more liberal theologies and open stances to new ideas and societal changes while maintaining traditional practices regarding their public gathering and church polity. They tend to be influenced by higher Biblical criticism, increasingly open to the ordination of women, and less dogmatic regarding issues such as homosexuality and abortion. In general, "Mainline churches" are less focused on doctrine. This, along with a lesser emphasis in soliciting new members, makes Mainline churches a diminishing percentage of overall Protestant adherents.
For example, the largest Mainline denominations in the United States (with more than a million members each)  are:
- United Methodist Church – 8,251,175
- Evangelical Lutheran Church in America – 4,984,925
- Presbyterian Church (USA)) – 3,241,309
- Episcopal Church – 2,320,221
- American Baptist Churches in the USA – 1,433,075
- United Church of Christ – 1,296,652
John Green, author of Religion and the Culture Wars, offers the following distinctions between Mainline and evangelical protestants:
Mainline Protestants have a different perspective. They have a more modernist theology. So, for instance, they would read the Bible, not as the inerrant word of God, but as a historical document, which has God's word in it and a lot of very important truths, but that needs to be interpreted in every age by individuals of that time and that place.
Mainline Protestants tend to also believe that Jesus is the way to salvation. But many mainline Protestants would believe that perhaps there are other ways to salvation as well. People in other religious traditions, even outside of Christianity, may have access to God's grace and to salvation as well, on their own terms, and through their own means.
Mainline Protestants are much less concerned with personal conversion. Although they do talk about spiritual transformation, they'll often discuss a spiritual journey from one's youth to old age, leading on into eternity. So there is a sense of transformation, but there isn't that emphasis on conversion -- on that one moment or series of moments in which one's life is dramatically changed.
Finally, mainline Protestants are somewhat less concerned with proselytizing than evangelicals. Certainly proselytizing is something they believe in. They believe in sharing their beliefs with others, but not for the purposes of conversion necessarily. The idea of spreading the word in the mainline tradition is much broader than simply preaching the good news. It also involves economic development. It involves personal assistance, charity, a whole number of other activities.