These hard-liners, taking advantage of the political damage that corruption investigations have done to Mr. Netanyahu, have staked out positions to the right of his. The prime minister was not even present at a meeting of the Likud leadership that for the first time urged the formal annexation of Jewish settlements in the West Bank. The Israeli Parliament, meanwhile, voted to require a two-thirds majority vote for any legislation ceding parts of Jerusalem to the Palestinians, raising an obstacle to any land-for-peace deal involving Jerusalem.

This should be the moment for the United States, Israel’s strongest supporter in the world, to step in and say no, that path can lead only to greater strife and isolation for Israel. But it is evident that for Mr. Trump and his son-in-law Jared Kushner, who is supposed to be leading the president’s Middle East efforts, diplomacy is a one-sided affair.

Furthermore, the threat to cut the substantial American contribution to the United Nations agency that supports more than five million Palestinian refugees and their descendants in Gaza, the West Bank, Jordan, Lebanon and Syria would foment a humanitarian crisis in refugee camps, threaten continuing Palestinian security cooperation with Israel and prompt more censure around the world.

Mr. Trump still claims he is in favor of peace talks. All he has done so far has been to create greater obstacles and fan the ardor of extremists on both sides. If he was really interested in a Middle East deal, as he claimed in his campaign, this would be a good time to reaffirm America’s longstanding commitment to a two-state solution and tell the Israeli right that it is going too far.