Tennessee Democratic Rep. Steve Cohen described being lobbied by passionate constituents and those on social media, who questioned his religion and called him a “kapo,” a term used to describe prisoners at Nazi concentration camps assigned to supervise forced labor. (Photo by Alex Wong/Getty Images)

Not since George H.W. Bush was president has the American Israel Public Affairs Committee 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 to its near-pristine political reputation.

In fighting the deal, AIPAC and its affiliates mustered all of its considerable resources: spending tens of millions on television ads in the home states of undecided lawmakers and organizing a fly-in to blitz legislators on Capitol Hill – another is planned for next week when Congress returns from August recess to vote on a resolution of disapproval. But all that noise amounted to a humbling and rare defeat this week, when President Obama secured enough backing in the Senate to protect the pact from efforts to dismantle it.

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Many say AIPAC’s efforts were doomed to fail in the aftermath of Israeli Prime Minister Benjamin Netanyahu’s combative speech to Congress in March — an appearance brokered by Israel’s ambassador to the United States along with House Speaker John Boehner (R-Ohio) without White House consultation. Many of AIPAC’s supporters also blame Obama and what they see as a process he rigged and a debate he polarized.

But whether the White House won a lasting victory in securing the Iran deal’s fate, AIPAC may have lost its claim to iron-clad influence over lawmakers on issues pertaining to Israel.

“The lesson that lawmakers have learned from this experience is that right-leaning pro-Israel groups are not immortal,” said Dylan Williams, vice president of government affairs for J Street, a more liberal, rival pro-Israel group. “Blood can be drawn. And it is possible to stand up and say “No” to them. And not suffer political consequences.”

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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 Obama has secured more than the 34 Senate votes needed to ensure that opponents won’t collect a two-thirds, veto-proof majority to block the deal. On Thursday, three more Senate Democrats sided with Obama — Heidi Heitkamp (N.D.), Cory Booker (N.J.) and Mark Warner (Va.).

[Graphic: Where lawmakers stand on Iran]

Undecided lawmakers found themselves in the crosswinds of a fierce maelstrom of political jockeying as the deal’s architects and opponents pressed their case.

“Vigorous and regular,” is how Sen. Chris Coons (D-Del.) described it.

Coons, who announced his support for the deal earlier this week, stressed that in Delaware, the lobbying he encountered from the deal’s foes was always respectful – in part because in that small state, the people conveying the anti-deal message were locals he has long known.

“But for other senators who have been comparably torn on this with whom I’ve spoken — where the ads in their states are much more aggressive than the ones here — it has backfired,” Coons said. “Instead of making them feel compelled to vote against the deal, it has made them feel resentful.”

Congress’s Jewish lawmakers came under some of the most intense pressure from anti-deal activists.

Rep. Steve Cohen (D-Tenn.), who announced his support for the deal in August, described weathering a barrage of attacks from passionately 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 labor – as they pressed him against the deal.

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

“Having him come and try to influence the members of the Congress and lobby against what the president was working on set the tenor,” Cohen continued. “Netanyahu should not get himself involved in American politics in the future, and AIPAC played a stronger hand than they should have.”

Other congressional aides pointed out that Israel’s unprecedented direct lobbying efforts against the deal by Israeli Ambassador Ron Dermer, who once worked for GOP operative Frank Luntz, worsened AIPAC’s position by association.

[Anti-Iran lobbying groups firing on all cylinders]

“There are a lot of people whose reaction to the Israeli ambassador’s lobbying was not a positive one,” said one Democratic aide to a Jewish lawmaker. “It wasn’t ever a harsh tone or bossy or threatening, but if you are looking at the group of Jewish Democrats, there could have been a better understanding of the nuanced approach those members were taking. Not ‘This is it, take it or leave it, and if you’re on the other side of it, you’re wrong.’ ”

The Israeli Embassy didn’t respond to a request for comment.

While AIPAC and Israel’s activities were not coordinated, some members of Congress felt the group was tacitly endorsing the Israeli government’s increasingly political line.

“They burned their bridges with Democrats before they got into this,” said a Senate aide who spoke on condition of anonymity in order to speak frankly about AIPAC’s apparent failure.

“They were silent and a little complicit in the Netanyahu debacle. They were just standing by when it happened. They spent down their political capital before they got up to this effort.”

[Iran deal ads running in at least 23 states]

Several Democratic lawmakers pointed to Netanyahu’s speech to Congress as poisoning the political environment surrounding the Iran deal debate even before an agreement was reached. 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 that Netanyahu’s appearance was inappropriate as Israel was preparing for national elections.

“The unfortunate problem with Prime Minister Netanyahu is that he prides himself on being the Israeli who knows America the best,” said former Democratic congressman Robert Wexler (Fla.), who now runs the D.C.-based S. Daniel Abraham Center for Middle East Peace. “Where he’s mistaken is, Prime Minister Netanyahu knows the America that elected Ronald Reagan president. He’s completely unfamiliar with the America that elected Barack Obama president. And they are in fact very different Americas.”

J Street, a rival lobbying group that spent far less money trying to persuade lawmakers to support the deal, said AIPAC’s lobbying tactics simply don’t work anymore.

“It used to be that AIPAC could deliver votes in a situation like this by emphasizing the political cost of going against them. That no longer works as well as it used to, with Democrats in particular, who recognize that the majority of their supporters in the Jewish community support this deal,” said Jeremy Ben-Ami, J Street’s executive director. “The days of AIPAC being able to present itself as the sole voice of American Jews on these issues are over.”

AIPAC’s executive director refused interview requests for this story. But AIPAC spokesman Marshall Wittmann said the group 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,” he said. “We believe that this strong opposition conveys an important message to the world – especially foreign banks, businesses and governments – about the severe doubts in America concerning Iran’s willingness to meet its commitments and the long-term viability of this agreement.”

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 characterized as a “loss.”

Middle East policy experts say that for an experienced lobbying organization like AIPAC, other factors were at play.

“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?”

Obama and Kerry have both said the United States will provide Israel more security guarantees, including military aid.

But others argue that if AIPAC wants to recover from this episode, it needs to mend fences.

“This will be a setback if AIPAC allows itself to become Republican-oriented, and allows the debate to continue in its partisan manner,” said Tom Dine, AIPAC’s former executive director until 1993. “The partisan debate got away from AIPAC, and it lost its bipartisan advantage.”

The Iran deal isn’t the first time that AIPAC has lobbied Congress hard and come up short. But the prior episodes to which experts and former lobbyists often refer – the 1981 sale of AWACS early warning aircraft to Saudi Arabia; and a fight in the early 1990s over loan guarantees to build housing in settlements in the Palestinian territories – all took place in a different political time.

Dine, who has ties to the Democratic Party and supports the Iran deal, said AIPAC mistakenly does not do enough to control members who say Obama’s policies have been bad for Israel.

“I just think it’s the dumbest thing possible,” Dine said. “Get with it, man! Go back to basics, and get those guys off the stage. AIPAC may not be able to control what a person says, but you can keep them off the stage.”

Clarification: This article has been updated to say that Israeli Ambassador to the United States Ron Dermer once worked for GOP operative Frank Luntz.