Speaking to reporters at a joint press conference in Washington, President Barack Obama and British Prime Minister David Cameron said on Friday that they opposed new sanctions against Iran until nuclear negotiations had run their course.

“I am asking Congress to hold off because our negotiators, our partners, those who are most intimately involved in this assess that it will jeopardize the possibility of providing a diplomatic solution,” Obama said. “I will veto a bill that comes to my desk.”

Domestic political pressures in both the U.S. and Iran have threatened to derail the work of negotiators in Geneva.

“All I’m saying is let us put in prospective sanctions that don’t get imposed…until July,” Senator Robert Menendez (D-N.J.) told reporters on Friday.

Melendez, who is the ranking Democrat on the Senate Foreign Relations Committee, is co-sponsoring a bill with Illinois Republican Senator Mark Kirk that would add new sanctions if a deal is not reached by the June 30 deadline. But that deadline is the result of a series of extensions that, along with their distrust of the regime in Tehran, has left many in Congress wary of the Geneva process altogether.

Despite the pressure from both Obama and Cameron, and the president’s argument that he would call for new sanctions should the negotiations fail, neither senator seems shaken in the conviction that a new sanctions bill is necessary sooner rather than later.

“If anything can stop Iran from getting nuclear weapons, it is maintaining the united bipartisan front in Congress to end Iran’s uranium enrichment and plutonium paths to the bomb,” Kirk said in a statement.

Iranian Foreign Minister Mohammed Javad Zarif told Time in 2013, when the negotiations had just begun, that new sanctions would kill a potential deal.

“We do not like to negotiate under duress,” Zarif said. “And if Congress adopts sanctions, it shows lack of seriousness and lack of a desire to achieve a resolution on the part of the United States.”

In a nod to that possibility, Obama on Friday said there was “no good argument for us to undercut, undermine the negotiations until they play out.”

“Congress needs to show patience,” he said.

Cameron, meanwhile, said he had contacted members of Congress “simply to make the point that as a country that stands alongside America in these vital negotiations that its our opinion that further sanctions won’t help.”

“We may not get there but we have a chance to resolve the nuclear issue peacefully,” Obama said, echoing Cameron’s concerns about new sanctions against Iran and adding that his “main message to Congress is: Just hold your fire.”

[photo credit: The Prime Minister’s Office]