It was a moment of striking, and for some people awkward, symbolism: Gary Samore, the president of one of the most prominent groups opposing the Iran nuclear deal, stepped down from his post last week because he had decided to support the deal.

Samore is a well-respected arms control expert; his decision to step down looked symbolic of the arms control community's overwhelming embrace of the agreement, and of the growing divide between that expert community and anti-deal groups. (I spoke with Samore last week about why he'd gone from a skeptic of the deal to a supporter, and about the reaction to his decision.)

What happened next seemed just as telling. The group Samore had led, United Against Nuclear Iran (UANI), didn't replace him with another arms control expert, but rather with former Sen. Joseph Lieberman. It looked like a sign that deal opponents had decided to stop fighting it on the technical merits and shift their emphasis to the politics. And it seemed significant that this avowedly bipartisan group had selected, as its new leader, a former Democrat who had turned against his party in 2008 to support John McCain's presidential campaign.

I was curious about the group's decision, and about how they're dealing with the changing political landscape around the Iran nuclear deal, so on Thursday I spoke to Mark Wallace, the CEO of UANI and a former Bush administration official, over the phone.

There was a lot I was curious about. Had they considered following Samore's lead in supporting the deal, and if not, why not? Why pick a former politician, and why Joe Lieberman? Had they affirmatively chosen to shift strategies from science to politics, or had they been forced into it by circumstances? How were they reacting to the growing trend of arms control and nuclear experts coming out in support of the deal?

We began by discussing UANI's case for killing the deal, as it is lobbying Congress to do. The US could blow up the deal, he said, but coerce Europe into maintaining sanctions by using the US financial system as leverage against European companies. Even if some countries dropped their sanctions, he argued, the low price of oil would act as a "force multiplier" on US sanctions, making up for any allies who dropped away. In time, he said, even if Russia and China dropped out of sanctions, we would get a better deal.

But was most interesting, I thought, was hearing Wallace talk about how this big, prominent anti-deal group was dealing with the changes in the political landscape — including the changes at UANI itself. If you read between the lines, you can get a sense of where the anti-deal movement sees its strengths and its challenges.

When we discussed Samore, Wallace was at pains to praise him and to signal respect for his decision. But he at one pointed suggested that Samore favored the deal in part out of "political loyalty" to the Obama administration, for which Samore previously worked as a nuclear expert.

When I raised Lieberman and why he seemed like an odd choice to persuade Democrats to side against their president, Wallace had good things to say about the former Senator, but seemed eager to shift the conversation instead to President Obama. Make of that what you will. What follows is a transcript of our conversation, lightly edited for length and clarity.

Max Fisher: The way that a lot of supporters of the nuclear deal are framing it, and the way that Gary Samore put it to me, is that this deal is better than any of the realistic alternatives. Can you tell me why UANI believes that's wrong, that there is a better, realistically achievable alternative?

Mark Wallace: What's overlooked is that the sanctions regime was always designed to get Iran to the negotiating table and to create leverage for our negotiators. If you look back at the height of the sanctions in 2012 and 2013, they were incredibly effective. Oil prices were at $110 for a barrel of oil. Oil sales coming out of Iran were somewhere around 2.5 million barrels per day, but it crashed down to a low of about 760,000 barrels per day. Right now, the price of oil has gotten in the $30s. That has the effect of being an incredible force multiplier on the sanctions. Iran would have to double its output of oil [to maintain oil revenues], which it couldn't. We could maintain even a good bit of sanctions architecture and use our diplomatic capabilities to keep our allies involved [in maintaining sanctions] to get a better deal. We're sanctions experts, and I assure you that with oil prices between $30 and $50 a barrel, the Iranians are fragile.

Max Fisher: A lot of people seem concerned that if Congress kills the deal, as UANI is advising, the other countries that are party to the sanctions will walk away, particularly non-allies like Russia and China or Asian countries, such as India and Japan, that signed on basically as a favor to the US. You actually worked, under the Bush administration, in the US mission to the United Nations. What lessons did you learn in the UN that lead you to believe the US could keep these countries on board with sanctions?

Mark Wallace: I actually think it's my experience after [working at the UN] that's more relevant here. At United Against Nuclear Iran, we've corresponded and spoken with hundreds, if not thousands, of international corporations, foreign ministries, world leaders to get them to forgo business in Iran. Nobody else has done that, and we've done it for years. And I would ask you a rhetorical question: Were the Chinese and Russians ever on board with sanctions against Iran? I would suggest probably not.

Max Fisher: They voted for sanctions, though.

Mark Wallace: They might have voted for them, but they were notorious trading partners with Iran. There were all those military arms sales. And Iran never viewed China and Russia as key trading partners. I think European trade is the more relevant question. There was always seepage from China and Russia. The bigger question is Europe. Even at the height of sanctions, German companies engaged in significant business in Europe. Germany had about $3.6 billion worth of trade, Italy had some trade with Iran. But the global financial system — for better or worse, but I think for better — flows through the United States, specifically New York. The lesson of 2008 was that if you are an international company and need to do business, you pretty much have to touch our financial system. Meaning that you if you did business with the country that is a number-one sponsor of terror and engaging in mischief in the region in the world, then we can say, as not only a military superpower but as an economic superpower, "If you do business, we think you shouldn't be able to do business in the US and shouldn't be able to access US capital markets." That's a very powerful statement. No compliance officer, no risk officer, no corporate official in any international conglomerate would have risked the reputational dangers and the risks to investments of going back into Iran if the US says we're not on board with this. The big difference between now and when we had sanctions before is that oil prices are far lower now. It wouldn't even take as strong of a sanctions relief as we had before in order to put Iran's economy in difficult circumstances. At $30 to $50 for a barrel of oil, that economy is very fragile.

Max Fisher: Let me ask about the changes at UANI this week. Gary Samore [who stepped down as president, announcing he supported the nuclear deal] has been known for some time as skeptical of the nuclear deal. I think some people were surprised back in April when the framework [terms outlining the basics of the deal] came out and Gary, very publicly, praised them as "solid." Was there discussion within UANI at that point that, if he decided he support the nuclear deal, that he should step down?

Mark Wallace: I don't remember any discussion like that. In April, I think we were all holding out hope for — we believed that no deal was better than a bad deal. I didn't want to prejudge things. I think that there was concern and skepticism by everyone on the trajectory of the negotiations, but I don't think anyone was prejudging anything. We were hopeful but there was real skepticism and concern. I think Gary shared that, we all shared that.

Max Fisher: When it became clear that Gary did support the nuclear deal, and thought it was, in his words, "better than the alternatives," was there any discussion within UANI of following his lead?

Mark Wallace: I think of Gary as a close colleague and friend, and we talk regularly. When the deal came out, we all wanted to read it, we wanted to understand it. UANI didn't make a snap judgment. Look, I respect Gary enormously. I think that Gary is of course conflicted, because he worked with this president, he worked on this team. People say that political loyalty is sometimes a bad thing, but I understand that conflict that was there. Gary sees the deal in a slightly better light than I do, but not a lot better light. And when you combine that, I think Gary came out in a close call supporting the deal. His words should speak to that, and I respect Gary; he's my friend, and I get that. I think the broader consensus and opinion on the deal was that there was a disappointment across a broad spectrum, from [former International Atomic Energy Agency official] Olli Heinonen to [president of the Institute for Science and International Security] David Albright to our staff and team. I think there was broad skepticism about it, and it became clear that it didn't meet the essential elements we'd been talking about. So we decided the right thing was to oppose the deal. So my first question was, "What's best for Gary?" And Gary felt that it was awkward for him to even support the agreement in a tepid manner as the president of UANI.

Max Fisher: Can you talk through how you settled on bringing in Senator Joseph Lieberman to lead the organization after Gary?

Mark Wallace: The senator was on our advisory board and we have a close group of people who have worked on this. He just seemed like the right person and wanted to come on, he's a good friend of mine and a man that a lot of people respect. He was delighted, and we were honored to have him.

Max Fisher: One of UANI's real assets has been having a leader with Gary Samore's credentials as a pedigreed arms control expert. Did you guys think about looking for a replacement who could also be seen as firstly an arms control wonk?

Mark Wallace: Well, Olli Heinonen, for example, is on our advisory board. I still have Gary's arms control expertise; I don't think that has changed. We have [head of the Nonproliferation Policy Education Center] Henry Sokolski. Olli Heinonen has been a big player in this. Even David Albright, even though he's not affiliated with us, has been very useful. I'm trying to label them all arms control wonks, you know what I mean; they're familiar with the space. We've been doing this now for a long time, and I think that the nature of the deal is quite well-framed. I don't think there's a lot of debate about the terms anymore. Certainly that expertise matters, but all of our team, we have a very seasoned team, but there are no secrets to that agreement anymore. Well, actually, there are some parts of the agreement that are secret, but you know what I mean. The terms of the agreement are well-baked, and there's enormous commentary on all of them. And you do see skepticism from the very serious people in that community, which I would characterize as Gary Samore, Olli Heinonen, and David Albright. I think their skepticism has been quite loud and clear about the agreement.

Max Fisher: With a couple of exceptions whom you named, the arms control community seems to have generally lined up behind the Iran deal. I think some people see the change in leadership in UANI as a sign that you guys are no longer focusing on trying to persuade the arms control community. Is that fair?

Mark Wallace: I don't see what you describe. When I think of the real experts who have led in the space, it's really been Gary [Samore], Olli [Heinonen], Henry [Sokolski], and [David] Albright. Those are leaders in that space. Olli's statements have been quite concerned about the deal, and even Gary acknowledges that there are some real problems with the deal.

Max Fisher: Okay, still, the fact remains that UANI will transition from being led by an arms control expert to being led by a politician. How should we read that? Is that UANI making a tactical decision to shift from focusing on technical issues to focusing on politics?

Mark Wallace: Well the president before Gary was Kristen Silverberg. [Silverberg served several positions in the George W. Bush White House and as Bush-era US ambassador to the European Union.] Gary was involved before that; so was Dennis Ross [who held senior diplomatic posts in the George H. W. Bush, Clinton, and Obama administrations]. UANI has always been a collection of real experts who are serious people, but our focus has really been on the sanctions architecture [against Iran], and I think we've really led in that space. I think we've always tried to be very serious about the arms control side of it, but we're not necessarily — we don't do the science of arms control, but we've had a strong selection of friends and colleagues from Gary Milhollin to Olli Heinonen to Gary [Samore] that've been there. But UANI's primary purpose was to help support the government and to lead to bring economic pressure. And I think we've been quite successful over the years, so I don't think that there's a departure from arms control. Gary remains on our advisory board, Olli Heinonen remains on our advisory board. I think quite highly of David Albright. I think we have that. And not to be critical of them in any way, their opinions are quite important here, but this is now also very much a political policy document. It's far from just arms control. I think they would be the first ones to acknowledge that it's not a typical — it should've been just an arms control document, but it's not. This has to do with sanctions, this has to do with conventional weapons. This has to do with a variety of things. So I think even they would object that this is purely for atom scientists.

Max Fisher: Politically, the fight right now is over Congress, which means that the fight is over Democrats in Congress and where they'll side. Gary Samore seemed like a good person to persuade Democrats, because he was an Obama administration alum and they know him. Are you worried that bringing on Lieberman, who is best remembered for vigorously supporting the Iraq War and betraying his party, is going to make that mission harder?

Mark Wallace: You don't mean that, I don't think that's right or fair, and I think that's an inappropriate way to characterize him voting his confidence. Senator Lieberman has served his country in many ways for many years, more than you or I combined, and with distinction. In reality, President Obama, if you go back and look at his statement during his campaign that he wants to bring an end to the childish, silly things and the partisan politics and bring principled leaders. And the statements emanating, when you see Ruth Marcus's piece in the Washington Post [criticizing Obama's rhetoric on those who oppose the Iran deal], I thought that was very compelling. I think you find that there are thoughtful people who are incredibly disappointed by the statements from this president and the demagoguery from the left. What's happening to Chuck Schumer [a Democratic senator who announced he will oppose the deal] is highly inappropriate. The mere fact that Democrats have to leave the party position in order to vote suggests they're putting party over principle, and I don't think we should be in that place. This is a really huge, important point. People should vote how they feel irrespective of party. Everything that I've seen suggests that people want principled decision-making over party loyalty.

Max Fisher: And Senator Lieberman is the person to help members of Congress through the principle-versus-party decision?