Not since George H W Bush was president has the American Israel Public Affairs Committee (Aipac) sustained such a public defeat on an issue it deemed an existential threat to Israel’s security.

But the Iran nuclear deal has Washington insiders wondering if the once-untouchable lobbying giant has suffered lasting damage. In fighting the deal, Aipac and its affiliates mustered all their resources: spending tens of millions on television ads in the states of undecided legislators and organising a fly-in to blitz Capitol Hill – another is planned for next week when Congress returns from its August recess to vote on a resolution of disapproval. But all that noise amounted to a rare defeat this week, when President Obama secured strong enough support in the Senate to protect the pact from efforts to dismantle it.

Many say Aipac’s efforts were doomed to fail in the aftermath of the Israeli Prime Minister Benjamin Netanyahu’s combative speech to Congress in March – an appearance brokered by Israel’s ambassador to the US along with the House Speaker, Republican John Boehner, without White House consultation.

Aipac’s position on the Iran deal lines up with the Republican Party’s, but its efforts thus far have helped persuade only two Senate Democrats, and a handful in the House – while Mr Obama has secured more than the 34 Senate votes to ensure that opponents won’t collect a two-thirds, veto-proof majority to block the deal.

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Congress’s Jewish politicians came under some of the most intense pressure from anti-deal activists. Representative Steve Cohen of Tennessee, who announced his support for the deal in August, described weathering a barrage of attacks from opposed constituents and others on social media, who questioned his religion, his intelligence, and called him a “kapo” – a term used to describe prisoners of Nazi concentration camps who were assigned to supervise forced labour – as they pressed him against the deal.

While he isn’t sure if Aipac could have improved the dialogue, Mr Cohen said “the tenor was set when Netanyahu came to speak to Congress without the President’s knowledge and/or approval”.

Some members of Congress felt the group was tacitly endorsing the Israeli government’s political line.

“They burnt their bridges with Democrats before they got into this,” said a Senate aide who spoke on condition of anonymity. “They were a little complicit in the Netanyahu debacle.”

Several Democratic politicians pointed to Mr Netanyahu’s speech to Congress as poisoning the environment surrounding the Iran deal debate even before an agreement was reached. Mr Netanyahu spoke to Aipac’s annual conference the night before his congressional address, arguing vehemently against the Iran negotiations. Several Democratic members boycotted the speech, arguing Mr Netanyahu’s appearance was inappropriate.

J Street, a rival lobbying group that spent far less money trying to convince politicians to support the deal, said Aipac’s lobbying tactics don’t work anymore.

“The days of Aipac being able to present itself as the sole voice of American Jews on these issues are over,” said Jeremy Ben-Ami, J Street’s executive director.

The Aipac spokesman Marshall Wittmann said it was aiming “to achieve the largest possible bipartisan majority that will reject this flawed deal. Many of the deal’s proponents have expressed severe concerns. We believe that this strong opposition conveys an important message to the world .”

And to many Aipac supporters, the fact that a majority of both chambers of Congress are expected to vote against the deal means the vote can’t be characterised as a “loss”.

“I suspect within Aipac itself there was probably a low expectation that they would succeed in this,” said Dennis Ross, a former White House Middle East peace negotiator. “So the question is, did they believe they could affect what the administration might do in terms of some of the commitments they might be prepared to make to Israel?”