For 40 years, Aipac, the American Israel Political Action Committee, has been the definitive lobbying voice for Israel in US politics.

A body with 100,000 members, Aipac has used its financial muscle and political skills to become arguably the most influential non-governmental voice on foreign policy in the US.

For most of that time, Likud-dominated governments have been in power in Israel and the organisation’s unquestioning promotion of Likud’s right-wing agenda — and Aipac acolytes’ harsh criticism of those who disagree — has left many US Jews, by inclination liberal, feeling alienated.

Slowly, though, Aipac’s supremacy is being challenged. Here are two recent examples.

On January 24, New York’s new mayor, Bill de Blasio, held an unannounced meeting with Aipac leaders. A reporter sneaked into the meeting and recorded it. Mr De Blasio told the group: “City Hall will always be open to Aipac. When you need me to stand by you in Washington or anywhere, I will answer the call and I’ll answer it happily because that’s my job.”

Within days, an open letter to the mayor was circulated to the Jewish press taking him to task for the meeting and reminding him that Aipac did not speak on behalf of all Jews. The letter was signed by prominent liberal Jews like Gloria Steinem, as well as several rabbis, among them Conservative Rabbi Burton Visotzky.

Rabbi Visotzky, also a Talmud scholar at the Jewish Theological Seminary, explained: “We were dismayed by Mayor De Blasio’s blithe assumption that Aipac spoke on behalf of the entire Jewish community.”

The letter had its desired effect. Mr De Blasio’s office said that the mayor would be “more transparent” in future in publicising his meetings with interest groups. This week, he submitted himself to a grilling on the Brian Lehrer radio show about the affair. Mr De Blasio reiterated strong support for Israel but did assert that he was not in lockstep with Aipac. He also brought up the other area where Aipac is being challenged. “I think the deal that has been struck with Iran is an incredible first step,” Mr De Blasio said.

Aipac was lobbying for new sanctions against Iran even as the first diplomatic steps towards getting that country to end its nuclear weapons programme are under way. President Barack Obama and Secretary of State John Kerry have said that new sanctions will not help the process.

Aipac did not care. It called on its members to let congressmen know how they feel. According to columnist J J Goldberg writing in the Forward, that meant one thing: bullying and threatening of representatives (that’s what lobbying at the highest level has become in America: the NRA does it as well) if they do not toe the Aipac line.

But Iran is not an easy issue to lobby on. “Diplomacy first” seems to Americans a better idea than it did in the aftermath of 9/11. And this week, Aipac announced that it would, for now, back away from lobbying for a new sanctions bill after it emerged that it did not have enough bipartisan support in the Senate.

Aipac is simply not having the impact it did a decade ago. Slowly, other voices are getting through. Rabbi Visotzky noted: “I think that the US Jewish community is more and more resembling the community in Israel, with a wide range of political opinion.”

Adam Dickter, assistant managing editor of the Jewish Week, notes, however, that the louder voices are still making the political running, which may explain why Mr De Blasio held that meeting with Aipac. “Politicians here tend to voice opinions that are somewhat to the right under the assumption that doing otherwise will provoke protests against them,” Mr Dickter says. Calls and emails to Aipac were not answered.

Michael Goldfarb is a journalist and author