Usually in conservative Protestant contexts when we talk about the difference between the evangelicals and the mainline, the differences in view are theological. Often they are actually more or less the differences articulated a hundred years ago in Machen's Christianity and Liberalism, even though today's mainline, particularly its younger members, tend to not really map well onto those debates. The errors of the contemporary mainline often (though certainly not always) have far more to do with anthropology than they do theology proper.
That said, those are not the only differences of note between the two sub-sets of American Protestant and while the theological divisions have been and will continue to be of immense importance, we can sometimes lose track of another set of differences that are also significant.
The overwhelming majority of churches that would be identified as "evangelical" today are downstream of the movements profiled in Nathan Hatch's essential The Democratization of American Christianity. These groups tended, in the period Hatch studied, to be driven by highly egalitarian impulses that caused them to be indifferent to theological training, historic liturgical practices and church polity, and so on.
In the paradigmatic case, Baptists and Methodists tended to spread across the frontier far more easily simply because if a young man came to them and said "I want to be a preacher," they'd say, "great!" and have him in the pulpit almost immediately whereas the Presbyterians and Episcopalians tended to say, "OK, move back to the northeast or New York or Virginia and spend several years in seminary and then you can come back."
The democratized forms of Christian belief profiled by Hatch existed on the frontiers of American society. Their adherents were outsiders, far from the elite bastions of the northeast. They tended to be highly individualistic, innovative, unconcerned with the past, and pragmatic in their overall approach to the Christian life. Crucially, this pragmatism was aimed at goals reflective of their own individualistic, egalitarian brand of Christianity—so it's revivalistic pragmatism more than social do-gooder pragmatism, you might say.
This style of religiosity has continued to define the movements we now describe as "evangelical" and "fundamentalist" down to the present. Though the two groups are quite different in many ways, both are downstream of Hatch's "democratized Christianity" rather than the old mainline. Thus even the notable differences between the two groups—evangelicals ordered more toward mission and outreach, fundamentalists ordered toward purity—both tacitly reflect the fact that both groups see themselves as outsiders and define themselves by how they navigate their outsider status.
Evangelicals, particularly as the post-war movement has defined itself, are not only more driven to evangelize, but are also generally wealthier than their predecessors (often with post-war Sunbelt money, to be clear, rather than through bringing old money into the movement) and often are at least somewhat more proximate to the halls of power than those predecessors. Neither of those conditions hold for the fundamentalists.
At their times of greatest success, the evangelicals then can actually cross over into the American mainstream in a way the fundamentalists never have in the post-war era. Obviously Tim Keller is one example of this phenomenon, but there are others we might cite as well: Hatch himself began his career as an undergraduate at Wheaton before eventually becoming a professor and later provost at Notre Dame and finally the president at Wake Forest. His friends and occasional co-authors, Mark Noll and George Marsden, followed similar paths: Noll studied at Wheaton as an undergrad and taught there for a time before moving to Notre Dame. Marsden taught at Calvin for 20 years before moving first to Duke and then to Notre Dame.
That said, because this crossing over is complicated and opens one up to new dangers and temptations, seasons of high crossover success for evangelicals often alarm and embolden the purity wing of the movement, as I described recently.
In contrast to this, the American mainline has never regarded itself as being sociologically outside the American mainstream. Rather it was the American mainstream to a large degree. In his excellent book An Anxious Age Joseph Bottum argues that American civic society for much of the post-Civil War era and especially in the 20th century was built on the three legged stool of the government, the market, and the church—by which he meant the Protestant Mainline. Bottum's claim, one which I found quite persuasive by the time I finished, was that the single most significant factor in the transformation of American civic society over the past several decades has been the collapse of the Protestant mainline.
The reason why isn't hard to understand. The American market generated wealth. The American government utilized that wealth to create power and security and opportunity. The American church, therefore, was necessary to provide moral guidance. She helped the market and government know what to aim for, you might say, or perhaps what not to aim for. The church, meaning the Protestant Mainline, was the moral conscience of the American social order.
As the Mainline has collapsed, no serviceable replacement has been found. Bottum's book highlights why the Roman church was unable to step into that role, even though for a very short time in the 1980s it looked like it might get there. Evangelicals, meanwhile, have been unable to overcome the outsider status that has marked them for 200 years since the days of their origins in antebellum revivalism.
The reasons for this inability are many.
The outcome of this is basically what is seen in today's public square: The moral conscience of the American system has completely vanished. As a result, the state and market operate with a general indifference to moral concerns. The group that should provide that voice, the old Mainline, is impotent and marginal and presumed to be irrelevant by most who hold actual power. The groups that might have provided it have failed to rise to the task. This means that the place of Christian belief in the American public square is uncertain and contested—which creates further anxiety for already generally anxious evangelicals. It also means that non-Christian people are left without any kind of moral roadmap for their life, which creates a crisis of meaning, identity, and belonging.
If evangelicals are to escape these problems, then the first step we need to make is not primarily theological or ethical, but is rather a kind of imaginative change in how we socially locate ourselves in the American republic. We need to be able to reckon with that republic as it actually is and how it is actually designed to function and we need to be able to navigate that particular social order with faithfulness, intelligence, and prudence.