When Daniel Ellsberg was 42, a judge threw out Espionage Act charges against him. At 90, he sought such charges again in hope of challenging their constitutionality.

In the last years of his long and remarkable life, Daniel Ellsberg, the disenchanted military analyst who famously leaked the so-called Pentagon Papers in 1971, wanted to be prosecuted. And he hoped I would help pave the way.

The charge he coveted was mishandling national security secrets under the Espionage Act, and his plan was to give me another classified document he had taken decades ago that he had held onto without authorization all this time. He wanted to mount a defense in a way that would offer the Supreme Court an opportunity to declare that law unconstitutional as applied to those who leaked government secrets to reporters. It is the same law former President Donald J. Trump is now accused of violating 31 times, though under very different circumstances.

Mr. Ellsberg’s disclosure in 1971 of the Pentagon Papers — a classified study of the Vietnam War showing that a generation of military and political leaders had lied to the public — and its fallout left a stamp on history that defined the bulk of his life.

But buried in some obituaries were glancing references to a 2021 episode in which he gave me a top-secret document about American military leaders’ push to carry out a first-use nuclear strike on China in 1958, accepting the risk that the Soviet Union would retaliate in kind on behalf of its ally, and that millions of people would die.

In examining his legacy, the scrutiny he tried to bring to the Espionage Act in making that disclosure also deserves attention.

“I will, if indicted, be asserting my belief that what I am doing — like what I’ve done in the past — is not criminal,” he told me, arguing that using the act “to criminalize classified truth-telling in the public interest” should be deemed unconstitutional.

The government has various tools to deter and punish unauthorized disclosures to reporters and the public, and for most of American history, it did not try to send leakers to prison. The Espionage Act has been on the books since World War I, but it was not until the second half of the 20th century that the government began trying to use it to charge leakers instead of spies — initially, to little success.

In 1957, the military included Espionage Act charges in the court-martial of an Army colonel for giving reporters information about a disputed missile program, but prosecutors dropped the charges. In 1971, the Justice Department obtained its first such indictment in the case against Mr. Ellsberg and a colleague who had helped him, Anthony Russo. But a judge threw out the charges, citing government misconduct and illegal evidence gathering.

A decade later, the Justice Department under the Reagan administration tried again, bringing Espionage Act charges against a defense analyst who had provided classified satellite photographs of a Soviet shipyard to Jane’s Defence Weekly. He was convicted. But it was so odd and unfair that only one person had been sent to prison for an act that had happened routinely for decades that President Bill Clinton pardoned him in 2001.

Starting midway through the George W. Bush administration and continuing under presidents of both parties, however, the Justice Department began routinely going after leakers using the Espionage Act. The law carries a harsh penalty — 10 years per count — and defendants are forbidden to suggest that juries acquit by arguing that their disclosures were in the public interest. Most defendants take plea deals to avoid the risk of lengthy sentences, foreclosing the opportunity for appeals challenging the constitutionality of using the law in such circumstances.

Mr. Ellsberg and I had talked about the government’s accelerating use of the law in 2014, when I wrote about how Edward J. Snowden, a former contractor for the National Security Agency who leaked secrets about surveillance activities, had joined the board of a nonprofit press freedom organization Mr. Ellsberg helped found. (Mr. Snowden, whom Mr. Ellsberg publicly embraced as a kindred spirit, has been living in Russia as a fugitive from Espionage Act charges.)

“The question — which hardly anyone realizes, I would say, is a question — is whether this application of the Espionage Act to people who are informing the U.S. public, not secretly informing a foreign power like a spy, is constitutional,” he told me. “The issue is hardly ever raised now. It wasn’t in my mind when I revealed the Pentagon Papers — I assumed I was breaking the plain language of that law, as I had been warned. And I was.”

In the years that followed, more reporters’ sources faced Espionage Act charges. And in 2019, the Justice Department under the Trump administration crossed a new line by obtaining an Espionage Act indictment of the WikiLeaks founder Julian Assange — not for leaking, but for soliciting and publishing leaks. The department under the Biden administration has kept those unprecedented charges; Mr. Assange has delayed a trial by fighting extradition in Britain, but keeps losing appeals there.

It was against that backdrop that Mr. Ellsberg called me on a sunny Saturday afternoon in the spring of 2021. At the time, I was watching a Little League game. I moved away from the low bleachers and lawn chairs to get out of earshot of other parents.

Decades ago, Mr. Ellsberg said, he had taken a second classified study based on internal government records, which he had not given to reporters at the time of the Pentagon Papers because it was about a different topic: when Communist Chinese forces shelled islands controlled by Taiwan in 1958, setting off a crisis. It showed that the world had come closer to nuclear war than the public had been allowed to know.

Was I interested in writing about it? I was.

In the weeks that followed, I carefully read the Taiwan study and consulted experts on the history of the 1958 crisis. As I worked on my article, Mr. Ellsberg and I spoke repeatedly. He preferred to talk by video call from his book-crammed home office in California. On one lengthy call, his wife, Patricia, joined us.

Part of his motivation, he said, was renewed tensions over Taiwan. He said it was likely that Pentagon war planners were again drawing up contingency plans to use nuclear weapons if China were to attack Taiwan and it appeared that conventional weapons were not enough to repel it. He thought the possibility of any such dire step warranted a public debate.

But another reason, he said, was that in openly confessing to retaining and disseminating the classified document without authorization, he hoped to be charged under the Espionage Act. He wanted to be a test case to put before the Supreme Court the constitutionality of how the Justice Department has used the law to punish leaks.

The provision against unauthorized retention of national security secrets, he pointed out, is so expansively written that, on its face, it could also be used to indict journalists, publishers and even readers of a newspaper article about a classified matter who tell a spouse about it or retain a clipping instead of turning it in to the authorities. Citing the chilling effect that the creeping expansion of the law has on what information the public gets in a democracy, he expressed disappointment that the Biden administration had not dropped the Espionage Act charges against Mr. Assange.

“It’s clearly overly broad and does not just apply to people like me who had a security clearance. Assange is now feeling the weight of that,” he said, adding: “For 50 years I’ve been saying to journalists, ‘This thing was a loaded weapon looking at you.’”

In short, at the age of 90, he was willing to risk the prison sentence he had been spared when he was 42. But as aggressive as the Justice Department has become in using the Espionage Act, it apparently wanted no part in going along with Mr. Ellsberg’s plan. The article published and drew some attention — but to his disappointment, no charges ensued.

“I was looking forward to arguing in court,” he told an interviewer in March, after he announced that he had been diagnosed with pancreatic cancer. “This is before I knew that my life would be shorter than I had expected.”