On foreign policy, Mr. Trump certainly uses muscular language. He says he would “bomb the hell” out of the Islamic State. That’s a welcome change from Mr. Obama, whose equivocations about the use of force have made America look weaker in the eyes of Israel’s enemies. And Mr. Trump has also been in line with the Netanyahu government in condemning the nuclear deal with Iran. But many of Mr. Trump’s statements on foreign policy suggest he has an isolationist streak. Last September, for example, he said: “Russia wants to get rid of ISIS. We want to get rid of ISIS. Maybe let Russia do it. Let them get rid of ISIS. What the hell do we care?” That poses a problem for Israel: The last thing Israel wants is an America that refuses to lead.

That is, except when it comes to Israeli-Palestinian relations, because if there’s anywhere the Netanyahu government might want to see a hands-off America, it would be on that issue. But here, Mr. Trump suddenly wants to take the lead. He promises he would broker a peace deal between Israel and the Palestinians and assures that he would do it as a “neutral guy.” For Israel “neutral” is a code word with one meaning: unsupportive. In fact, “neutral” is how Israelis tend to describe Mr. Obama’s policies by way of expressing displeasure. There are other ways Mr. Trump echoes the Obama administration: He has hinted that the lack of progress on the Israeli-Palestinian conflict has to do with Israel’s lack of enthusiasm rather than, as Mr. Netanyahu and most Republicans believe, because the Palestinians aren’t a serious partner for peace.

And then, of course, there is Mr. Trump’s unpredictability. While previous presidential candidates did not always support policies that were to Israel’s liking, most of them had a fairly comprehensible agenda: Mr. Obama wanted to get a deal with Iran, to pull out of Iraq and to promote peace between Israel and the Palestinians. Whatever the Netanyahu government thought about his policies — and it disliked most of them — Mr. Obama almost always tried to do exactly what he promised. Mr. Trump, on the other hand, is vague about his plans, if he has any. Sometimes he promises to destroy the Islamic State, other times he wants to leave the task to Russia. He criticizes the Iran deal, but unlike some other Republican candidates, doesn’t say he will “rip it up” if he is elected.

In recent weeks, Mr. Trump has repeatedly called himself Israel’s “biggest” and “best” friend. When it comes to Israel-United States relations, friendship is a word with many definitions. Early in his presidency, Mr. Obama said that his role as “a friend to Israel is partly to hold up a mirror and tell the truth” when he disagreed with its policies. It’s no wonder that these days, the American president and his Israeli counterpart often look more like adversaries than friends.

Israel’s government hoped a new president would restore a more traditional definition of friendship — one based on mutual trust and support. But if the choice comes down to Mrs. Clinton or Mr. Trump, that’s unlikely. From a President Clinton, Israel’s hawkish government would come under more of the same pressure it received from the Obama administration. From a President Trump … anything is possible.