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According to Mr. Kimball, the key questions now on the table involve precisely how far the Iranians are willing to go in limiting their enrichment; how much international oversight they are willing to allow; and what specific sanctions relief the West is willing to offer in return.

The amount of wiggle room the Obama administration has with regard to sanctions relief may depend on its relationships both with Congress and with Israel.

Heading into this week’s negotiations, six Democrats and four Republican lawmakers wrote a letter to President Obama calling for an all-out halt of uranium enrichment activities by Iran. The lawmakers, including Sen. Bob Menendez, the Democratic chairman of the Senate Committee on Foreign Relations, also called on the Obama administration to threaten an increase of sanctions if Iran does not move quickly toward positive negotiations.

The letter dovetailed with a stark warning issued this week by the government of Israeli Prime Minister Benjamin Netanyahu against moving too quickly toward lifting the sanctions.

In a statement Tuesday, Mr. Netanyahu’s security cabinet asserted that the Iranian leaders have “systematically defied” past U.N. Security Council resolutions calling for an end to uranium enrichment activities. The statement also urged world powers to accept nothing less than a full suspension of such activities before any sanctions are lifted.

At the Arms Control Association, meanwhile, Mr. Kimball said the push by Israel and by some in U.S. Congress for a total halt of uranium enrichment is “unrealistic” and may jeopardize the prospect for a breakthrough in current negotiations with Iran.

“As a matter of national pride,” he said, Iranian leaders are “going to insist upon some limited level of safeguarded enrichment.”