Last December, when I interviewed the leader of Israel’s left-leaning Labor Party, Isaac “Bougie” Herzog, at the Brookings Institution’s Saban Forum, he said, in reference to nuclear negotiations with Iran: “I trust the Obama administration to get a good deal.”

In a telephone call with me late last night, Herzog’s message was very different. The deal just finalized in Vienna, he said, “will unleash a lion from the cage, it will have a direct influence over the balance of power in our region, it’s going to affect our borders, and it will affect the safety of my children.”

Iran, he said, is an “empire of evil and hate that spreads terror across the region,” adding that, under the terms of the deal, Iran “will become a nuclear-threshold state in a decade or so.” Iran will take its post-sanctions windfall, he said, and use the funds to supply more rockets to Hezbollah in Lebanon, more ammunition to Hamas in Gaza, and “generally increase the worst type of activities that they’ve been doing.”

Herzog, who lost a race for the prime ministership in March to the Likud’s Benjamin Netanyahu, had mainly kind words for his archrival, and he even invoked an expression popularized by Netanyahu’s ideological guide, the founding father of right-wing Zionist revisionism, Ze’ev Jabotinsky, to describe what he sees as Israel’s next, necessary step: “We have to build an iron wall to protect Israel. There are clear risks to Israel’s security in this deal.”

The Iran deal represents one of those rare issues that has unified Israelis of most political parties. Herzog and Netanyahu agree on very little—not on a whole basket of social and economic issues, and certainly not on the need for territorial compromise to advance the cause of a two-state solution with the Palestinians. But Iran, Herzog told me, has Israelis—of the “left, center, and right,” he said—frightened.

Netanyahu appears eager to bring Herzog, the official head of the Israeli opposition in the Knesset, into his government as foreign minister. This makes good sense from Netanyahu’s perspective—he knows that he has burned bridges with the Obama administration, and he needs an interlocutor who could gain access to the West Wing. Herzog wouldn’t tell me the status of his talks with Netanyahu, though he said he believed he could do a more effective job critiquing the Iran deal from outside the government. And this is where things gets complicated: When I asked Herzog if he would be lobbying Congress to disapprove the deal (AIPAC, I’m told, has invited him to do so), he said he wouldn’t. “I think it’s a bad deal, but I’m not going to lobby, I’m not going to tell senators what to vote. I think what I need to do is explain the weak points and have them understand our concerns. I’m taking the practical approach.”

Isn’t that a description of lobbying? “I don’t intend to hide my feelings. Most of the Israeli body politic is worried about the agreement, and people need to understand our worries. The world doesn’t fully understand the fact that we are left here alone in this neighborhood, that there is a Shia empire that is trying to inflame the region with a heavy hand. But I don’t intend to clash with the administration. We’re very glad for all that the Obama administration has done for us. We have respect for the United States, for this great ally and friend, and we don’t want to be in a confrontation or clash. But we have to let people know that we think this is a dangerous situation.”

Herzog’s militancy on the subject of the deal places the Obama administration in an uneasy position. While the administration can—and has—dismissed Netanyahu as a hysteric, the eminently reasonable Herzog, who is Secretary of State John Kerry’s dream of an Israeli peace-process partner, will find receptive ears among Democrats for his criticism. Herzog’s critique of the deal also places American Jewish organizations in a curious dilemma. It will be fraught for liberal Jewish organizations to endorse the Vienna agreement if both the right-wing government in Jerusalem, and its center-left opposition, are so vehemently opposed to it. (The only major Jewish organization to line up with the Obama administration so far is J Street, which describes itself as “pro-Israel and pro-peace,” but which is keenly interested in advancing Obama administration interests, whether or not Israelis agree with them. “Our No. 1 agenda item,” its founder, Jeremy Ben-Ami, once said,is to do whatever we can in Congress to act as the president’s blocking back.” Herzog, J Street’s natural ally in Israeli politics on matters of the peace process, is putting the group in an uncomfortable position. It can’t be easy to be a self-described pro-Israel group that is lobbying for a deal that the large majority of Israelis loathe.)

Herzog would not tell me when he’s arriving in Washington to launch his non-lobbying lobbying campaign, but I expect he will arrive soon, and I expect that he will find himself the target of a great deal of lobbying as well; from the administration’s perspective, Netanyahu is a permanent adversary, but Herzog is a respected friendone who could do damage to the administration’s cause on Capitol Hill, if he so chooses.