Part of the difficulty for both party establishments is that Trump and BLM are very light on policy and heavy on emotional appeals. Both focus on how broken our country is. For Trump it’s “the American dream is dead,” for BLM it’s “the country is based on and steeped in white supremacy.” As many have pointed out, Trump’s appeal is based on burning down the establishment, so is BLM’s. In both cases this can lead to a hopelessness among their supporters that is poisonous to the positive message presidential candidates must pronounce.
There is a long time until Election Day, but right now it is difficult to see hardline Trump supporters embracing Bush or Rubio. BLM activists don’t seem much closer to closing ranks with Clinton or Sanders. There are two essential demographics in play here.
For Republicans, the working class, white male was the missing link in 2012. A segment of the population that the GOP must motivate more effectively than Romney did. This group is Trump’s bread and butter. For the Obama era Democratic coalition, black turnout made the difference between presidential victories and midterm congressional shellackings. Right now, BLM has enormous sway over this group.
It is the direct appeal to these respective demographic groups that scares the party establishments.