To be more specific here, mainliners have no memory, or maybe in some cases a selective memory, about the degree to which the mainline was once a frequent, some times even a primary perpetrator of many of the same things for which mainliners now get peeved at evangelicals for doing.
For example: do you think that the only pastors to whom Rev. Martin Luther King Jr. was writing from the Birmingham jail were evangelicals? Uh, no. While a few mainliners managed to stand with the civil rights movement, many, many more were either keeping silent or chiding King for making trouble or wanting to move too quickly.
I am reminded of the kerfuffle over the "new" book by Harper Lee that somehow appeared late in Lee's life, Go Set a Watchman, in which Atticus Finch (the hero of To Kill a Mockingbird) was a rather less than heroic figure. I'm not sure which was more upsetting to many folks: the original perception that Go Set a Watchman was a sequel, suggesting that we were finding out that a beloved literary character was less wonderful than previously thought, or the later realization that GSAW was in fact an early version (a first draft, if you will) of TKAM.
For me, I think, the later version is ultimately a more depressing version of events. One could easily get the idea that Lee more or less wrote what she saw and experienced, raw as it was, and sent GSAW for potential publication with Atticus as the more flawed character she had probably known in many of the men she had known and seen as a youth, only to discover that readers of the late 1950s/early 1960s needed to have the story "sweetened" to make it palatable; they required a good guy, and specifically a white good guy. (This is of course leaving aside more mundane editorial issues that needed to be addressed in GSAW.)
Modern mainliners may be guilty of this to some degree.
It's not hard, for example, to be in Presbyland and hear of Eugene Carson Blake. A Presbyterian minister and educator, later denominational and ecumenical leader, Blake holds a place of high regard in mainline Presbydom for having stood with and even marched with King and other civil righs leaders in the March on Washington, and speaking at the Lincoln Memorial. I don't know how many other folks outside Presbyland would know of him, but he is highly regarded within this particular domain.
As far as I can learn and discern, Blake was legit, a throughgoing progressive on race and many other matters. But we moderns shouldn't necessarily assume that Blake was necessarily representative of all of the then-PCUSA or UPCUSA denominations in which he served.
So yes, Presbydom and the mainline in general can take some measure of solace in Blake's work. But the mainline also has some real pieces of work in its various family trees, like one James Fifield. A Congregational minister who fought like, well, hell to prevent the merger that created the United Church of Christ, Fifield is particularly noteworthy as the "Apostle to Millionaires," embracing without apology and with horrible exegesis what would seem like a proto-prosperity gospel, except that its aim was far more sinister, and far more directly corporate. It wasn't just prosperity for which Fifield shilled, but specifically the prosperity and profitability of American tycoons. The "Christian America" rhetoric of today (like this) has its roots not in anything of the colonial era, but in Fifield's work.
Note: I have already recommended Kevin M. Kruse's One Nation Under God: How Corporate America Created Christian America, in which Fifield figures prominently. This NYTimes article from a couple of years ago summarizes some (but not all, not nearly) of the salient points of that book.
In short, if we're gonna claim our Blakes, we gotta own our Fifields. We gotta be forthright about the role we played in how supposed Christianity in this country got to the point where it is today. A lot of the stain might be on evangelicals now, but they didn't do all the work.
There won't be any successful going forward (whatever "forward" turns out to be) until we do that reckoning.
Rev. James W. Fifield, Jr., the "Apostle to Millionaires," or "St. Paul of the Prosperous"