The pact between Mr. Netanyahu and the Kahanists set off a predictable eruption from liberal Jewish groups like J Street and Americans for Peace Now, as well as the Union of Reform Judaism, which normally stays out of Israeli politics.

But the outrage was not limited to the left.

On Friday, the American Israel Public Affairs Committee, the American lobbying group known as Aipac, and the American Jewish Committee, both of which rarely weigh in publicly on Israeli politics, declared Otzma Yehudit’s ideas “reprehensible.” They vowed not to have any contact with its leaders even if they become part of the next government.

In an equally extraordinary step, Rabbi Benny Lau of Jerusalem, a pillar of religious Zionism, repeatedly assailed the merger over the weekend, warning on social media that “the defilement and destruction of the land serves as a guarantee for the loss of the land.”

Rabbi Lau lamented that the prime minister seemed concerned only with winning re-election, and, from his pulpit at the Ramban Synagogue, likened Kahanism to Nazism and its ideas to the Nuremberg Laws.

“The entry of the racist doctrine into the Knesset is the destruction of the Temple,” he wrote on Facebook on Sunday.

In Israel’s chaotic parliamentary system, small parties like the ultra-Orthodox Shas can be make or break when it comes to forming a majority coalition after an election, and Mr. Netanyahu has routinely struck deals giving them outsize influence.