I believe many of his supporters don't think this matter is all that important -- that it has no bearing on his capabilities as President. But I think it does. If the US makes agreements with other countries, is president Trump going to suddenly decide to welch on those agreements? We already have a shaky relationship with our allies that must be repaired. I suspect a Trump presidency will leave the US friendless.
This is just one of many character flaws which would be a deal-breaker, for me.
He has a tendency to attack first, get facts later, and if the facts don't agree with his actions, he'll lie and double down on the attack. (The way he dealt with the Liz Mair PAC ad)
He will lie about people, commonly hanging a derogatory nickname on them, if they are in disagreement with him. ("Dopey Prince Al-Waleed" was one long before he went up against Cruz, "Crooked Hillary" another after). Such childishness might play with the masses in a campaign, but the core of it is a need to feel superior to his opponents, belying someone who is far less secure than they project.
His constant shifting of positions (If it isn't in writing and signed, it isn't a deal, and then the lawyers can break it), sometimes within hours (as with his stance on Common Core). Not something to hang national policy on.
His stiffing of subcontractors merely provides an indicator that if he feels he will gain, screwing the little guy is okay. Making the little guy take a partial settlement in exchange for the promise of more work (Duh. You screwed me once and you want me to go back for seconds?) is a common tactic. His 'moral compass' points in one direction consistently, toward himself.
Does anyone think he will take his multinational corporation and put it in a blind trust and then act in the Country's best interest, even if it means his assets lose value overseas? It isn't as if he won't know where those assets are. Will his actions be influenced by the effect those actions will have on the value of those assets? If the leaders of ISIS were having a convention in one of his hotels, would he order it bombed to rubble, knowing the insurance would not pay off on an act of war declared or undeclared?
The need to occupy the spotlight as shown by the dramatic and planned interruption of Cruz' speech and the orchestrated booing at the convention. Not only did this present himself in a glorifying light to his supporters, but was designed to humiliate a former opponent instead of using the statement by that former opponent to promote his campaign. Trump took a possible advantage and opportunity to heal the schism in the party and instead, produced a festering sore in his quest for personal revenge because Cruz would not endorse him.
That, somehow, he felt entitled to that endorsement bespeaks a need to be placed on a pedestal, held in high esteem, even worshiped on Trump's part, which exposes a hungry ego that craves adulation. The response to that lack of adulation, an attack--even when that did not best serve long-term goals and Trump knew Cruz would not endorse after Trump's attacks on Cruz' family--bespeaks not just a sudden anger response, but one which was considered and orchestrated, planned well in advance of its execution. The job of President of the United States comes with no guarantee of ego-reinforcement, in fact, it is one open to serious attack, from the domestic press and foreign sources as well. Trump mentioned the weakening of the First Amendment, and with his need for constant glorification and to attack those who vilify him or are even simply neutral, what will become of political discourse during a Trump Presidency if he can't handle criticism?
A "more efficient" Federal Government is one of Trump's selling points. We don't need more efficiency so much as we need less Federal Governance in our private lives, and for the Federal Government to enforce the laws it has already in re the border and national security. It isn't a question of efficiency, but the will to do so. Downsizing the Government, reducing its scope and returning the power usurped from the States and the People to its rightful place will help balance the Federal Budget. More government will not. More efficient Government will continue to damage American industry, more efficiently. That won't Make America Great Again.
Finally, pandering to the Ethanol lobby to garner favor of the Governor in Iowa (mere hours after Cruz had come out against the mandate, invoking the ire of the Governor whose son is/was an ethanol industry lobbyist), at the expense of those who own older vehicles, small engines from outboard motors to leaf blowers and generators, and motorcycles/ATVs, and 10% of the fuel mileage most of America gets, while claiming he will use the one Agency which has done more damage to American Industry than any other to the "fullest extent of the law" to enforce that mandate.
It is said, "No candidate is perfect.", but did the Republican Party have to pull one out of the hat who so thoroughly exemplifies that?