The case is simple and can be summed up in one sentence. America should support Israel because they have a positive view of America and are longstanding Allies.
In the Israeli conflict we have two sides. The Palestinians who are at the very least affiliated with some jihadis who actively hate America and have no relations with us and Israel. In 2015 81% of Israelis viewed America positively and Israel has been an American ally for the longest time.
The rationale is simple. We want to encourage more countries to be longstanding allies and to view America favorably and we want to punish countries who don’t so we can encourage the desired behavior from them. Does it mean that we should always blindly support our allies? Of course not. However the presumption of support must be given to our allies and that presumption must be stronger the longer the alliance lasts. For example if we had a new ally who was borderline in support towards us then a 60/40 case would be sufficient for us to consider the other side. For a longer ally like Britain or Israel a 70/30 or 80/20 case should be considered. In the case of Israel and Palestine, while both sides have valid arguments, they are close enough that our presumption should remain with Israel.
The counter argument is of course, would we not alienate more countries like Palestine in our quest to reward our allies? The answer is no. If we apply this consistently then nations will see there is a tangible benefit to becoming a long-term American ally and to popularize America with its population instead of demonizing it. For instance countries like the Philippines which love America should get much more foreign aid than Afghanistan or other Middle Eastern countries that hate it.
The reward MUST come after the change in behavior and not before. When you discipline a child do you reward him before he does the right thing? Or do you reward him after he gets good grades, does his homework, or a host of other things? The end result of this method of foreign policy should be an international community competing to who can be the best, most cooperative ally to be assured of US support. It is not a novel concept either. Supporting your allies and punishing your enemies have been around since ancient times. In fact the chinese have a saying “Do not trample over your old friends in your rush to make new”.
At the end of the day the issue itself is only tangential to how our decision is to be made. Unless the case is absurdly one-sided, which it is not in this case, then we must stand with our allies so we can create new allies in the future.