Sen. Bob Corker (R-Tenn.) chairman of the Senate Foreign Relations Committee, attends a hearing on the Iran nuclear agreement, which is widely opposed among Republicans. (Andrew Harrer/Bloomberg News)

Diplomats from the five countries that negotiated the Iran nuclear agreement with the United States have launched a coordinated lobbying effort on Capitol Hill, with some warning lawmakers that if Congress scuttles the accord, there may be no chance of resuming talks to get a better deal.

“The option of going back to negotiations is close to zero,” Philipp Ackermann, the deputy ambassador of the German Embassy, said in a briefing Thursday with reporters.

Ackermann and the deputy chiefs of mission from the other nations involved in negotiating the Iran deal briefed 25 Democratic senators on Tuesday, an unusual session in which diplomats from Britain, Germany and France were aligned with counterparts from Russia and China.

A second meeting is planned next week with more senators.

The diplomats have been in regular contact with members of the House in advance of a September vote on the nuclear agreement with Iran. The GOP-led Congress is expected to reject the deal reached last month in Vienna. But President Obama has vowed to veto any attempt by Congress to block the deal, and the White House has been working to get enough Democratic support to prevent a veto override. Republicans are expected to vote as a bloc against it, so the diplomats are focusing their outreach on Democrats.

Highlights of the Iran deal

The intense lobbying surrounding the Iran agreement is unusual because of the involvement of so many foreign governments, including Israel, which is adamantly opposed to the deal.

In recent weeks, Israeli Ambassador Ron Dermer has met with dozens of undecided Democratic lawmakers to argue that the agreement is weak and poses an existential threat to Israeli security.

The diplomats of the P5+1 — which comprises the United States and the other five nations involved in the negotiations with Iran — have echoed many of the arguments that Obama has made, including during a speech this week aimed at bolstering support for the accord.

The diplomats say they want to back Obama and make it clear that much of the world agrees with him. But their efforts suggest that they are not entirely confident that the vote can withstand a veto override.

And Republicans have cited the support from Russia and China as a sign that the deal is not in the United States’ interests. “China and Russia have never been on our side of the table,” presidential candidate Carly Fiorina said during a GOP debate on Thursday.

Ackermann said: “The prospect of the rejection of the deal makes us nervous. In Germany, there is no debate. Not in Parliament, not in civil society. Everyone thinks it’s a good deal.”

The focus of the meeting with senators earlier this week was on what happens if the deal does not pass Congress and a presidential veto gets overridden. Every diplomat who attended the briefing said international sanctions would collapse, Iran would ramp up its nuclear program and there would be no possibility of getting Iran back to the negotiating table, diplomats said.

British Ambassador Peter Westmacott said he has met with at least three dozen members of Congress to discuss the deal, either by himself or within his colleagues from the P5+1.

“Since the deal was reached, I have been on the Hill and on the telephone talking to members of both houses,” he said. “We believe that it is a good, verifiable deal and the best way of stopping Iran from getting nuclear weapons. All the alternatives are worse.”

Diplomats also told senators that many European nations will not go along with continued sanctions if the deal is derailed by Congress. “It brings us back not just to square one, but before square one,” Ackermann said.

He also said he expects the International Atomic Energy Agency to address new questions about the Parchin military site, where some governments suspect that Iran conducted research more than a decade ago in preparation for the possible development of nuclear weapons.

Satellite images taken since the nuclear agreement was signed on July 14 show construction vehicles and containers being moved. The photos, taken from commercial satellite images, have raised suspicions that Iran is trying to scrub incriminating evidence from the site before IAEA inspectors have a chance to examine it, as agreed to in the nuclear deal. Unanswered questions about Iran’s possible weaponization research must be satisfied before sanctions can be lifted.

State Department deputy spokesman Mark Toner said Thursday that radioactive materials cannot be scrubbed away. “There are traces that remain, and we are confident in our ability to be able to fully investigate,” he said.

In a statement, Iran’s mission to the United Nations said it “strongly denies the baseless claims about the so-called clean-up operations in the Parchin Military Complex.” It said the images showed road repairs being made.

“The Islamic Republic of Iran . . . has never had any military nuclear activity and has never been engaged in any unconventional act that would need a hasty cover-up.”