The way I read the Dershowitz Precedent is this:

It will be generally assumed from this point forward that abuse of power is not possible, on the part of any President, so long as his or her actions are carried out in an attempt to be re-elected. The underlying principle put forth by Dershowitz is that every President believes their election to be in the nation's best interest and therefore any action taken in furtherance of this aim is legal.

Think of it this way: US Special Forces killed Osama Bin Laden on the orders of President Obama in the interest of protecting this nation. Few protested this action as unconstitutional or illegal. Trump could now, by the Dershowitz Precedent, conceivably have the Democratic nominee for President killed on the grounds that his or her election would threaten US security by preventing him (Trump) from remaining as President. Yes, it's that serious.