I think that it refers to the controlling elite of the Democratic Party (who are far from being democratic) who decide what the party thinks. This is why despite enormous support for Bernie Sanders and Elizabeth Warren, it is highly likely that they will not be chosen to run against Trump.I saw that Obama has voiced against them now. This is because the elite want someone who does not attack corporations nor billionaires. The elite actually support much the same things as the Republican Party. I write as a Canadian observer confessing that we have exactly the same problem. We have two right wing parties that alternate holding power. As long as this situation continues in both Canada and the USA, we have no hope of solving inequality nor Global Warming. The elites love the way the money is pouring into their coffers.

rogerbenham pretty much sums it up. Establishment is a referral often used towards the 2 party system we have in the US, especially by dissenting opposition to their ways of governance. Deep State would be the reference towards the ruling class the establishment works for. The ruling class is made up of the most wealthy in the country, corporations, bankers, wall street, some add the intelligence agencies to this also. The ruling class are vastly the major beneficiary entities to our current laws and passing legislature today. Often with corporations playing a part in developing our laws today through lobbying and pay to play schemes. The energy corporations and healthcare industries are vastly known as major players in this that make the news most often. Chemical companies are among the top also. Banking institutes.

If you follow politics closely enough you'll find people from these entities in every presidential administration who end up having great influence on every presidents policies for the past several decades. These entities manage to get their millionaire and billionaire people into administrations to curve policies to their benefit then return to their profession to reap the rewards when that administration ends or they achieve their goals. In which they often do (achieve their goals) unless they are caught on unethical issues or other questionable matters are brought out about them. Almost every president in recent history ends up with someone from a banking institute or wall street as their financial adviser. They leave a very lucrative million dollar a year profession to serve in a administration for a couple of years or more, then go back to that profession and returning to their yearly income again almost certain to get millions in bonuses for their time in the administration and accomplishments.

Trump has the most questionable administration in the history of our country in every aspect of our governance! In which both establishment parties allowed to go into place with little lip service in compliance. What little progress we were making towards the environment, education, homelessness, poverty, housing, veterans, healthcare, immigration, add your concerns here, we are now going backwards because of these types of ruling class people and entities.