In an interview in Iran in mid-March, Mr. Aboutalebi said he had been the interpreter for the Vatican’s special representative when he visited the embassy. He added that “one or two other times” he had done translations into English or French, including interpreting at a news conference two weeks into the hostage crisis when the occupiers decided to release 13 hostages.

“It was based purely on humanitarian motivations,” Mr. Aboutalebi said of his involvement.

There is no evidence that Mr. Aboutalebi served as a regular interpreter or translator or participated in interrogations of the hostages.

During the crisis, President Carter called the hostages “victims of terrorism and anarchy.”

So he could be forgiven for seeking vengeance against Iran today. Instead, Mr. Carter has called on the United States to move on. “Those were college students at that time, and I think that they have matured,” he said on a recent radio program, adding, “It would be inappropriate for the United States to try to block someone that Iran wanted to choose.”

But saying no to Iran over an ambassadorial choice comes at no political cost in Congress, so it was easy for both houses to vote unanimously to prevent Mr. Aboutalebi from entering the United States. And on Friday, the White House said it would not grant Mr. Aboutalebi a visa, effectively scuttling the nomination.

It remains unproved whether Mr. Aboutalebi was “an acknowledged terrorist,” as Senator Ted Cruz, Republican of Texas, who sponsored the Senate bill, branded him, or even a “major conspirator” in the hostage crisis, as Senator Charles E. Schumer, Democrat of New York, called him.

What is true is that the hostage crisis was an improvised affair that evolved over time. The leaders of the takeover had, as I had assumed, planned it as a sit-in that would last at most a few days. Even Ayatollah Khomeini’s initial instinct was to liberate the embassy, and it was only when he saw the masses of enthusiastic demonstrators at the front gates that he decided to support the occupation.

Image Hamid Aboutalebi, a political adviser, is Iran’s nominee for United Nations ambassador. Credit... President’s Office, Islamic Republic of Iran

The demands and threats of the hostage-takers — and Ayatollah Khomeini — quickly escalated. First it was the return of the deposed Shah Mohammed Reza Pahlavi, who had been allowed into the United States for medical treatment, then the return of Iran’s financial assets abroad, including the shah’s personal wealth.