It's actually quite simple. Both elite Republicans and Democrats hate Trump. Trump is a populist with conservative leanings. That scares both elite camps. Why, because he changes the fundamental nature of the GOP party platform to one that is more populist. A focus on the average American, the forgotten man.
So Trump will cut taxes, mostly for middle America. He will cut corporate taxes to stimulate job and economic growth. He will cut regulations for the same reason. He will repatriate overseas profits to extract concessions from businesses to invest in America. He will spend money on infrastructure (GOP doesn't like, Dems love). He will rebuild the military. He will focus on inner cities with tax credit zones for business investment, for education choice, for active policing to reduce crime, for stimulus spending. He will renegotiate trade agreements to reduce our trade deficits and focus on what's best for Americans, which unions will love. He will build a wall. He will secure the border and stop the flow of cheap labor which unions will love. He will embrace an all of the above energy strategy, which unions will love.
This will cause a realignment of both parties. The GOP becoming the champion of middle America, and his other policies designed to break the strangle hold on the black vote. Even Hispanics, who are culturally conservative, may warm to Trump if he "legalizes" the roughly 11,000,000 non-criminals who have a job.
That is the realignment both party elites fear. The GOP elite hate some of these policies. The Dem elite fear they will work. If they do, Dems lose a lot of union support as well as minority support.