Just last week, as the previous round of talks with Iran came to a close, a senior American official involved in the negotiations said that the framework accord with Iran would have to be more than a political declaration of intentions. Rather, it would have to contain a “quantifiable dimension.”

There is a lot to quantify, from the number of uranium-enriching centrifuges that would remain spinning to exactly how Iran would change the design of a reactor that is under construction to limit the production of plutonium, another pathway to a bomb. But Iran says it will not agree to such specifics, at least for now.

“This is one of the biggest challenges we face,” one European diplomat involved in the talks said in recent days. “The politics in America demand specificity, and an Iranian commitment. And the politics in Iran demand vagueness” and no commitment until a possible final deal — with all its technical annexes — is reached in June.

The European official added, “All of us are in agreement that you don’t make oral deals with Iran.”

Secretary of State John Kerry met with President Obama at the White House on Tuesday, administration officials said, in part to give the president the latest details before departing for Lausanne, Switzerland, on Wednesday to continue talks.

At the core of the problem is this political reality: Republicans in Congress, along with a significant number of Democrats, took the March deadline for a political agreement announced by Mr. Kerry on Nov. 24 as a critical milestone for any accord. If the Iranians could not provide specifics by then, many in Congress told the White House, then it was a sign that Iran was deliberately dragging out the process and needed to be further pressured by new sanctions.