I think the very best we could hope for is complete and utter gridlock along with actual responsible parties running around trying to perform damage control. Meanwhile, rogue pieces of his administration will all be vying for their own pet projects and causing more damage than those that are attempting to repair it can handle. (Look at his current administration and the foul up of the GOP platform to get an idea.) Trump has shown that he has absolutely NO understanding of the presidency, the executive branch, it's powers and scope, or of our constitution. Actually, he has the civics understanding of a child that believes the president is equatable to a king and can do anything he wants. The really sad thing is no one around him has taken the time to even explain a little of this to him to make it clear.
His big stupid mouth has already caused anxiety around the world and within our country, causing minor damage, and he hasn't even been elected yet. Regardless of what you think of our foreign policy - politically, militarily, and economically, the world depends on us being predictable to a certain degree for global stability. Trump will throw all of this into flux. Our allies will be much less confident, while our enemies will think about causing us and our allies harm in the ensuing chaos of such an administration. There's a reason Putin wants Trump to be president, and it isn't because Putin actually thinks Trump's dumb ass is a "genius", I'll tell you that. If you're against war, Trump is NOT your man. Even if we somehow beat the odds and do not get into direct conflict with someone militarily (Yes, Trump seems protectionist, but he lies as a matter of course) others will seek to impose their will militarily without a strong and reactive US looking over their shoulder. The military will be wary of a commander and chief that has already indicated that he would ask the military to do things against the law and the military code of conduct. The world's most powerful military without a strong leadership component should scare anyone.
Trump has also proven himself to be dangerously misguided when it comes to justice and how the US should act, suggesting we kill terrorists families and use torture for example. With our president saying such things, even IF he doesn't try to get the military or intelligence services to comply with such policies, such rhetoric will inflame the Muslim world against us even further, causing a greater incidence in attacks, which will in turn put Trump in a position to intensify our counter-attack, this will cause an unjustified response and lessen our respect around the globe even lower than when GW Bush was President.
Others have already pointed to the evidence that a Trump administration would cause a YUUGE uptick in our deficit. Our economy, still shaky, will most likely run back into recession. This will in turn force global markets back down, reversing gains made by other countries as well. Very shaky markets: Greece, Portugal, Spain, etc. won't fair well.
Domestically, I would expect racial tensions to flame up, as the racists, already feeling emboldened by a presidential candidate that thinks and speaks like they do now feel as if they are running the show again, and the US is back on track to the 1950's. What were once opinions that were often relegated to dark corners will often see the light of day again. With Trump's authoritarianism comes more cop butthole sniffing, and minorities will suffer disproportionately more than ever. This will cause rises in demonstrations and riots. Trump, ever the entrepreneur, will authorize more military hardware to the police and even further privatization of the penal system to compensate for the influx into the prison system. More rights infringed, more communities devastated without fathers and mothers being present in their communities. More money being pumped into incarceration.
Do I think we'll all go up in a mushroom cloud? Not really, but it won't be pretty.
