Should mainstream Christians take a stronger political stance?

Yes. Absolutely. By not speaking out against the far-right conservative Christians of this country, mainstream Christianity (whether that that be casual, liberal, non-denominational, etc.) is seen as silently supporting them. I'm not saying that is indeed the case, but that is the impression it leaves. A message has to be sent to this fringe that they have to join modern society in the 21st century and enter the political arena to defend the Constitution and not their religious beliefs. As an atheist, I'm not sure citizens like myself hold enough sway to convince them of this, but practitioners of mainstream Christianity might be more convincing and seen as less of a threat.