New Iranian president Hassan Rowhani.

GENEVA: The United States and Iran head into Thursday negotiations full of growing optimism that a deal can be reached on the future of Iran’s nuclear program. One of the key reasons for this optimism is the apparent willingness of the US Congress and of major American Jewish groups to refrain from demanding new sanctions against Iran for now.

The United States has apparently won a two-month pause from four U.S. groups it met with last week, including the powerful American Israel Public Affairs Committee (AIPAC) and the Anti-Defamation League. A source close to the talks told Breaking Defense that Washington pushed for a longer pause but that agreement had been reached, despite public protests by AIPAC that they would not back off in their lobbying Congress for sanctions. Israel remains supportive of American diplomatic efforts, as it has been for years despite its protests that Iran is just playing for time while increasing its nuclear capacities.

This opens the way for a sustained diplomatic push. “What we’re looking for now is a first phase, a first step, an initial understanding that stops Iran’s nuclear program from moving forward for the first time in decades and potentially rolls some of it back,” a senior US official told reporters. This first step, after a decade of fruitless negotiations, would hopefully clear the way towards “a comprehensive final agreement that will resolve all questions about Iran’s nuclear program,” including whether the Islamic Republic has done weapons work, the official said.

The Iranian take had a similar ring to it. “We are prepared to reach an agreement,” Iranian foreign minister and chief nuclear negotiator Javad Zarif said in a television interview. “We need to see an endgame that we can all agree with and we need to take a first step mutually on all sides so that we address the immediate concerns of the various sides in these negotiations. I believe it’s not that difficult to reach that agreement. I believe it’s even possible to reach that agreement during this meeting (this week in Geneva),” Zarif said.

This brightly optimistic tone from both sides is striking. There has, after all, been no real progress since the new era of US-Iranian diplomacy began in September with the first direct contacts between senior officials from the two nations since the Iranian revolution in 1979. Iran has managed during a decade of talks to build up a nuclear program with over 18,000 centrifuges installed, with over 10,000 actually spinning with uranium gas for enrichment, and with stockpiles of thousands of pounds of enriched uranium. Fundamental differences remain over what the first step will look like, let alone the shape of a final agreement. The bottom line is that Iran does not want to suspend uranium enrichment, the process which makes what can be fuel for civilian power reactors or the explosive core of atom bombs. The United States, leader of the six nations negotiating with Iran, wants to see enrichment reined in enough to guarantee that the Islamic Republic cannot “break out” and in a dash make enough weapon-grade uranium to produce a nuclear weapon. (CORRECTED “Tons” to “pounds” of enriched uranium. Nov. 7 at 10:52 a.m.)

Zarif insisted that continued enrichment for Iran must be part of any final solution and that sanctions that have devastated Iran’s oil sales and ability to trade internationally must be lifted. But the US focus, the senior US official said, is for a first step that would “halt their program from advancing further.” Then, with Iran’s nuclear work under control, a final solution could be sketched out.

The U.S. official said Washington was “prepared to offer limited, targeted and reversible sanctions relief. We are not talking about touching the core architecture of Iranian sanctions regime in this first step in any way.” And if a comprehensive, final agreement is not reached “any economic relief we will have given Iran can in fact be reversed.”

The goal in a first step is to “address the level of enrichment, the stockpiles of enrichment, the capacity of their facilities, the verification monitoring, all of those elements must be undertaken and resolved in a first step.” This would “put time on the clock” by halting Iran’s nuclear progress, which would leave room for talks. The period of time for this first phase that has been most discussed is six months, the U.S. official said.

That pause would include no new sanctions against Iran, the Americans say. Heightened punitive measures would be “harmful” and risk derailing the negotiations. But, as Iranian officials must know, in the end “we’re all pro-sanctions” if Iran proves to be recalcitrant, the US official stressed.

Michael Adler studies the Iranian nuclear program and non-proliferation at the Wilson Center in Washington.