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“A Pollard release would certainly make Netanyahu’s day,” Mr. Miller said, but would risk angering many in Washington.

Advocates note that some of those convicted for spying for American enemies have served far shorter prison terms.

Even after all these years, many details of the case remain a mystery. Pollard delivered piles of documents to his handlers, and their full extent has never been disclosed.

Pollard’s lawyer, Nitzana Darshan-Leitner, said the material traditionally had been shared with the Israelis. She said the information included details about Arab and Soviet military capabilities, and it helped Israel to assassinate a senior PLO official in Tunisia in 1988.

Ms. Darshan-Leitner said Pollard suffers from diabetes, high blood pressure and kidney stones.

“His medical condition is very precarious,” she said.

A recently declassified 1987 CIA damage assessment concluded that Pollard did not spy against America. According to the version published in December by the National Security Archive, a George Washington University project, Pollard cooperated “in good faith” while in custody, and his handlers’ requests were limited to intelligence on the Pakistanis, Arab states and the Soviets.

Pollard was granted Israeli citizenship in the late 1990s during Mr. Netanyahu’s first term. While Mr. Netanyahu was out of office, he visited Pollard in prison. Last year, Mr. Netanyahu made a formal appeal to the United States for Pollard’s release and a personal plea to allow him to attend his father’s funeral. The requests were denied.

For years, the Americans resisted an early release by saying it would harm national security. The stiff punishment also was seen as a deterrent to allies, warning them of the consequences of spying on American soil. But opposition has begun to crack, raising hopes that a breakthrough may be near.

Former Secretaries of State Henry Kissinger and George Schultz, along with Lawrence Korb, the assistant secretary of defense at the time, former CIA Director R. James Woolsey, Sen. John McCain and former Vice President Dan Quayle have all called for Pollard’s release in recent years.

Pollard’s wife, Esther, said that given these developments and the passage of time, the moment was ripe for Mr. Obama “to right this injustice.”

“I pray that President Obama will respond without any further delay,” she said in an email message.

• Steven R. Hurst in Washington contributed to this article.