White House arm-twisting may have secured the votes to force the Iran nuclear deal through a hostile Congress, but European ambassadors in Washington are getting credit for playing an outsized role.

In the stifling heat of early August in Washington, ambassadors from Britain, France, Germany as well as Russia and China sat down with around 30 Democratic Senators who held the fate of the landmark deal in their hands.

The envoys from disparate countries had united to bring one message: Don't kill this deal.

"It's not often you hear those five ambassadors all saying they agree with each other," said one British diplomat present. "I think the demonstration of unity behind this deal was really striking."

The collective presence of four high-profile figures -- Britain's Peter Westmacott, Germany's Peter Wittig, France's Gerard Araud, as well as the EU ambassador David O'Sullivan -- would come to be a feature of European lobbying efforts.

Since an interim agreement was signed with Iran on July 14, European officials had begun to fear an historic opportunity to curb Tehran's nuclear program -- in return for sanctions relief -- would fall victim to US domestic politics.

The Republican controlled Congress would almost certainly vote to reject the agreement, and Obama would certainly veto that rejection.

What the White House and the European nations needed was enough votes in favor -- one third in either the House of Representatives or the Senate -- to uphold Obama's veto.

- 'A powerful formula' -

"It was concerning for us that the deal could be prevented from being implemented by a Congressional vote," said the British diplomat, "pretty quickly we wanted to be out there explaining why it was important to us."

Each embassy would have their own lobbying effort: Westmacott alone spoke to 45 members of Congress.

But they often found it useful to forge a collective front.

"Once you realize the formula worked, that the four ambassadors sitting down with the same Senator or group of Senators is a powerful formula, then you deploy it as much as you can," said one senior European diplomat.

"If you look at the four people involved, they have very different personalities, but they actually make up an impressive high-caliber group."

Each brought their own experience: Westmacott and his deputy had previously served in Tehran, Wittig and Araud were ambassadors at the United Nations and O'Sullivan had spent three decades as a power player in the choppy waters of Brussels, trying to unify competing agendas within the EU.

They quickly pinpointed individual Senators and Congressmen who could be persuaded, or could bring along another couple of votes.

But, another European diplomat said, "once it became understood that we were out there we were actually approached by staffers and the officers of Congressmen and Senators to ask for our view."

Sometimes the task was to provide technical details, other times it was to shoot down the notion the Europeans were acting on behalf of the White House.

"There was some suggestion that the reason you have the four ambassadors trotting around the Hill was because the White House picked up the phone," said one diplomat. "It was in our interest. It was our duty to sell the deal."

"Obviously we would compare notes with them sometimes... but it was very much on our own back," said a British diplomat.

Part of the envoys' task was also to dispel the notion that Paris, London and Berlin would never reimpose sanctions if Iran cheats.

"We were really keen to say 'yes we would, and actually we have implemented sanctions more recently and taken more of an economic hit and we are ready to do it again'."

- 'No, seriously' -

But the most forceful argument -- and the one cited by eventual deal supporters Senators Bob Casey, Chris Coons, Barbara Boxer and Kirsten Gillibrand -- was the lack of alternatives.

"We had the opportunity to say 'no, seriously, this is the best option," said the British diplomat.

"'We've had our top people on it for a very long time now... you are not going to see Iran come back to the table, you won't see international unity and pressure like we have.'"

It was about "striking down that idea that if Congress said no to this we'd all just kind of shrug our shoulders and say 'back to Vienna it is then'."

British Ambassador Peter Westmacott recalled: "The more I spoke to members of Congress, the clearer it became that there were good answers to their questions and concerns, and that although the deal is not perfect, there is no better way of preventing Iran from acquiring nuclear weapons."

While difficult to quantify the role the European lobbying effort played in securing the 34 votes needed to uphold Obama's veto, a US administration official praised it as helpful.

They "underscored the point that the world was united behind this deal. And that any attempts to scuttle it would have tremendous implications for our standing in the world."

"It was fanciful to think that the coalition would come back together if the United States scuttled it."