Using a word like coup was intentional.
History starts everyday. Any study of the past show time and again that change happens slowly (unnoticed) then all at once (usually with the people wondering how it happened)
Perhaps you think its not possible to undermined democracy and turn it into a new world order. Yet such things have been done in the past
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You think the system and Constitution will protect you from any misuse of such a force, maybe, but its not a sure thing, history has shown that playing with this type of fire tends to end badly.
Being someone who has had close contact with the "tin foil hat" crowd since 1992, has been sympathetic to many of their claims, and even somewhat agreeable with other concerns they've raised in years past. I can fully relate. Many of the people you're arguing against probably "get it" better than you do. The "world" you now find yourself in is strange and frightening. Welcome to where many of them were 8 years ago, or where a number of people went back in 1992(and have remained) after George H. W. Bush gave his "New World Order" speech followed by Bill Clinton becoming PotUS in '93.
Yes, Democracies can be hijacked and overthrown, as we've seen happen internationally numerous times. Even the United States itself can be "hijacked" to various ends, as demonstrated by John Adams, Andrew Jackson, Woodrow Wilson, FDR, and even Barack Obama as PotUS. Congressionally, we've even seen it happen with McCarthyism that I can recall off-hand.
Things can get bad when you're "under the thumb" of an opposing President with sufficient support, but history has also shown that it takes a LOT throw the Federal system off course. Also, the thing the Anti-Trump people are failing to realize is the long-term trajectory of the Federal Government has been towards the modern incarnation of "liberalism" (which scares me, frankly), and as such, while "Trump is scary" freaking out in his first month is overkill. Not even Glenn Beck was this hysterical in Obama's first month in office. While he may have gotten close to being this hysterical(regarding Obama) at points during Obama's first 2 years, I actually don't think he made it as far as "the left" has already gone with Trump.
So that said, Trump raises a LOT of valid concerns, they're currently likely to only be short-term issues. As meaningful, lasting, change in the nature of the Federal Government is likely to require a decade or more of sustained control of the reigns of state. A cooperative congress during that time also helps considerably.
My concern is that "the opposition" (in the form of the Democrats/left-wing) are burning themselves out "in pursuit of the wrong things" right now. In some respects, I'm glad of that, because I don't like many aspects of their own respective agenda, so the sooner they render themselves functionally inert, the better. But in others, its bad because it leaves me wondering what the alternatives will be in 2 years.
Their focus should be on organising, educating, and preparing for 2018. Not on creating an ongoing political spectacle of unending opposition and protest to Everything Trump(tm).
But as it stands, early outcomes look to me like it'll likely be the Conservative/Libertarian(rather than reactionary/religious) aspects of the Tea Party acting during the Republican Primary races that will likely end up being the true backstop to anything particularly nefarious Trump may get up to. The Democrats and the left-wing still doesn't seem to "get it" but maybe they're still working through their 5 stages of grief.
Of course, the Libertarian approach to things further terrifies the Democrats on many fronts, ironic given the complaints right-left just voiced. Considering the Democrats are the one who typically advocate for ever increasing amounts of centralized (federal) control over things. The very same thing that is a pre-requisite of (quickly) implementing a tyrannical regime.
You don't build a build a very effective tyrannical regime by undermining your own power base by devolving control of things back to the citizens, or local communities(/states), themselves.