That was an interesting question, as to should the United States guarantee Israeli security as it does Japan, in which the candidates answers were only semi-answers, if that is for understandable reasons.
Of course, given Zionist and Evangelical constituencies in the United States, they practically fell over one another promising to defend Israel from any attack, which is fine, if it evaded what was thrown out there as to why Israel doesn't have a formal guarantee like Japan, and NATO for that matter in Article Five of the NATO Treaty, that if Israel were to be attacked, America would respond with anything, possibly tactical or strategic nuclear weapons.
What they didn't do was address why that idea is such a weighty one, which of course because Japan is not an acknowledged nuclear weapons state, while in effect Israel is, which gives Israel more autonomy in its foreign policy than Japan, ceteris paribus.
(Japan has 1,000 kilograms of plutonium, and since there is a most definite limit to "gaijin's" ability to penetrate elite Japanese circles, it is assumed by many that Japan could if it wished construct nuclear weapons rapidly should it feel that America would not defend it against a nuclear armed China, which is what the security relationship is largely though not exclusively oriented towards.)
As to Israel and the question of why no formal guarantee, that is because both sides have always shied away from that, if there has been an advocacy lobby for making Israel a member of NATO (and the EU) as part of a general Middle Eastern peace settlement.
America has Arab allies which have feathers that might, or might not, be ruffled if there were such a guarantee, although if that was done in the context of a settlement over the status of the Palestinians, and the fatal issue to date, Jerusalem, those feathers might well be smoothed over, as to why America in part has hesitated in extending such a formal nuclear guarantee, if most assume that there is an implicit guarantee.
(The other main American motive in shying away from that has been a concern about getting dragged into Israeli ventures in which America has a different view as to interests, as in Lebanon at times, if not only that.)
As to Israel, Israel has shied away from such an arrangement because of a fear of loss of autonomy, particularly since to make such guarantees credible, usually America has placed "tripwire" forces of sufficient magnitude, as in Korea, Germany, Japan, and the United Kingdom, so that potential adversaries would believe it quite possible that the military might insist on saving fellow soldiers lives by risking nuclear war, thereby making deterrence credible.
It is curious on that note to observe that in Prime Minister Netanyahu's speech to Congress last year, he explicitly stated that America was lucky to not have to deploy troops to Israel, if left unstated is the autonomy provided by Israeli nuclear capabilities to burn Moscow and Beijing among other places most definitely to the ground.
Of course, sine we are focusing on Iranian nuclear programs, having that discussion isn't considered expedient, although again, there have been those who have argued as has the author that extending a nuclear guarantee, plus three brigades for each flank as tripwire forces, in the context of a general Middle Eastern settlement would be feasible in theory as to peace, if anyone thinks of course anything but war is going to ultimately come out of the region.
But, that was the best question of the night for sure.