In the end, though, investigators never determined who pilfered and turned over to the Ronald Reagan campaign the briefing books President Jimmy Carter was using to prep for his 1980 debate with candidate Reagan. No one fell, and debategate got filed away decades ago in the musty museum of political trivia, under the heading of unsolved mysteries.

Now, in his new book, “Rendezvous With Destiny: Ronald Reagan and the Campaign That Changed America,” a much broader look at the campaign that elected Reagan, Craig Shirley, Reagan biographer and conservative public relations executive, has gathered a wealth of evidence, all of which points to one man — the late political operative and onetime Kennedy family confidant Paul Corbin — as the culprit.

There’s no smoking gun in the book — but there’s lots of gunpowder and interviews with people to whom Corbin admitted that he was the one.

In the summer of 1983, the city of Washington was in an uproar. A scandal erupted when it was revealed that just days before the 1980 presidential election, someone had stolen top-secret debate-briefing books from deep inside Jimmy Carter’s White House and given them to Ronald Reagan’s campaign.

When the news broke, the FBI and a congressional subcommittee set out to discover who had stolen the briefing books. The congressional investigation, headed by Michigan Democrat Donald J. Albosta, took 10 months, cost $500,000, interrogated hundreds of witnesses and eventually produced a report totaling nearly 2,500 pages. Yet the panel never officially determined who had pilfered the documents.

To this day, President Carter is deeply upset about the purloined material. “I don’t think there’s any doubt that it made some difference,” he complained when I interviewed him. Before the debate, held just a week before the election, he and Reagan were in a dead heat; Reagan ended up winning the election in a landslide. But who stole the documents has remained a mystery.

Now, almost 30 years later, the answer can finally be revealed: It was Paul Corbin, a Democrat from the 1980 Ted Kennedy campaign, who orchestrated the theft of the Carter briefing books and gave them to Reagan’s campaign.

•••

How did Paul Corbin, a lifelong Democrat, labor organizer and former Communist agitator, end up working for the ardently anti-Communist Republican Ronald Reagan? The answer lies in the fact the diminutive, shadowy Corbin was a political troublemaker par excellence.

An FBI file opened on Corbin in 1940 — only four years after he entered the country illegally from his native Canada — had swelled to nearly 2,000 documents by the time of his death in 1990. Papers released by the FBI under a Freedom of Information Act request reveal that Corbin spent much of his life one step ahead of the law, grand juries, the FBI and union thugs. He was arrested multiple times, including twice for running a scam in which he threatened businesses with strikes unless they bought ads in union publications: The publications were fraudulent, and he pocketed the ad money.

Corbin was also a card-carrying member of the Communist Party (registration number 62908). Nevertheless, in the early 1950s, he did brisk though surreptitious business with the biggest anti-Communist headline seeker in America: Sen. Joe McCarthy (R-Wis.). As McCarthy barnstormed the Midwest in the early 1950s, Corbin (as he later told reporters and friends) would go into towns in advance of McCarthy’s appearances and sell American flags to the people eagerly awaiting “Tail Gunner Joe.” Corbin and the senator split the profits.

Corbin was loyal to almost no one except the Kennedy family, and especially Robert Kennedy. His relationship with RFK began to flourish in 1960, when they both worked on John F. Kennedy’s presidential campaign. As attorney general, Bobby wanted to get Corbin a job in his brother’s administration. But RFK’s principal assistant, John Seigenthaler, reviewed the massive FBI file on Corbin and, as he recalled in an interview, told the attorney general, “You can’t hire him! You can’t hire him!” Instead, at RFK’s instructions, Corbin was quietly put on retainer with the Democratic National Committee. Kennedy even gave Corbin the key to his private elevator at the Justice Department.

Few understood Bobby’s close friendship with Corbin. Old Kennedy hand Joe Dolan called Corbin “the dark side of Bobby Kennedy,” according to Jeff Shesol’s book “Mutual Contempt.” RFK’s daughter Kathleen Kennedy Townsend admitted that her “Uncle Paul” was “a rascal” who “didn’t respect people,” but she told me she understood what her father saw in Corbin: “My father appreciated somebody who would find out what’s going on in the government or in politics and would be forthright about telling him so that he could have eyes and ears in places that he wouldn’t normally have them.” Columnist Drew Pearson characterized Corbin’s role more bluntly: He was “Bobby’s backstage henchman.”

When RFK was assassinated in June 1968, Corbin was shattered. But he got back into politics after moving to Tennessee. Corbin later bragged to some friends that he had the goods on Democrat Al Gore. Joseph Sweat, one of Corbin’s associates in Tennessee, remembered that Corbin accused Gore, then a young congressman, of renting rooms in a motel in Cookeville to watch pornography. “That goddamn Gore, he is up there in a motel ... watching those dirty movies!” Corbin exclaimed. Asked how he knew, Corbin replied, “The desk clerk; I paid him a little bit, and he gave me the receipt.”

In the late 1970s, Corbin went back to Washington. Held at arm’s length by the Carterites, he grew to hate Carter. Naturally, he supported Ted Kennedy’s presidential bid in 1980. He couldn’t stand watching Kennedy concede the party’s nomination to Carter at the Democratic National Convention in August. As Kennedy spoke, Corbin stormed off the convention floor. His friend Bill Schulz of Reader’s Digest called after him, asking what his plans were now that Kennedy was out of the race.

Corbin yelled back, “I’m going to go work for Reagan!”

•••

Just before Ted Kennedy officially launched his presidential candidacy in the fall of 1979, Carter’s ambassador to Mexico, Pat Lucey, abruptly quit his post and announced his intentions to help Kennedy.

Lucey was good friends with Paul Corbin. They had become close back in 1960, when Lucey, then Wisconsin’s Democratic Party chairman, had brought Corbin in to work on JFK’s campaign.

Lucey’s daughter, Laurie, had been working in the Carter White House at the time as a confidential assistant to Landon Butler, deputy to chief of staff Hamilton Jordan. But just days before her father quit his post in Mexico, she resigned from her White House job. She, too, was close with Corbin, who had taken her under his wing.

Around the same time that Laurie Lucey was leaving the White House, Bob Dunn was coming in. Dunn went to work for Carter’s head of scheduling, Phil Wise. Dunn was Pat Lucey’s longtime aide, having worked for him in Wisconsin and in Mexico. And Dunn was Corbin’s friend. Corbin had cultivated the young activist since 1971. In fact, it was Corbin who helped Dunn get the job on [former Wisconsin] Gov. Lucey’s staff.

“Dunn’s new job has attracted some attention since Lucey joined the campaign for Sen. Kennedy,” National Journal reported at the time. But the Carter team took no action.

•••

According to David Keene, a senior adviser to George H.W. Bush who frequently played poker with Corbin, and Adam Walinsky, an old RFK hand who was friends with Corbin, it was Corbin’s idea to persuade Pat Lucey to go on the ticket with independent candidate John Anderson. The thought was to bleed more liberal votes away from Carter. Lucey didn’t need much convincing after Kennedy was out of the race, as he loved Kennedy and despised Carter and his aides. In late August, Anderson presented Lucey as his running mate.

Corbin also contacted Reagan campaign manager Bill Casey. The two had been introduced soon after the Democratic convention by syndicated columnist Charles Bartlett, a fixture on the Washington social scene. Corbin proposed to assist Reagan, ostensibly with organized labor. Casey agreed, putting Corbin on retainer to the Reagan campaign.

Corbin first visited the Reagan-Bush headquarters in Arlington, Va., on Sept. 29, meeting with senior campaign adviser Jim Baker and then with Casey. He would make at least three more visits, signing in on Oct. 11, Oct. 25, and Nov. 3.

On Oct. 25, Corbin signed in at 9:35 a.m., gave his destination as “Casey” and picked up a check for $1,500. It was just three days before the big debate between Reagan and Carter.

On Nov. 3, the day before the election, Corbin picked up a second and final check from the Reagan campaign, this time in the amount of $1,360. He also spent nearly two hours meeting with Casey.

Carter’s debate briefing books had been assembled and copied in the White House starting the night of Oct. 23 and finishing around 11 o’clock the next morning. Copies of the briefing books arrived at the Reagan campaign’s headquarters not long thereafter. Reagan adviser David Gergen later recalled a package arriving at the Reagan-Bush campaign on a rainy Saturday, “probably Oct. 25” — the same day that Corbin met with Casey.

The “debategate” scandal didn’t explode until 1983, when Laurence Barrett of Time reported in his book “Gambling With History” that someone in the Reagan camp had “filched” Carter’s briefing material. A number of figures came under suspicion in the resulting investigations but were never charged. Paul Corbin was one. Congressman Albosta’s committee was unable to pin Corbin down. Frustrated, Albosta told The New York Times, “He denies everything, ... doesn’t even know his own name. This leads people to suspect he had some effort and involvement.”

Furthering suspicion was the fact that, as phone logs revealed, Corbin had called Bill Timmons at the Reagan campaign “on several occasions.” The Albosta committee reported that Corbin was informing Timmons of Carter’s travel in advance, including stops scheduled for “a week-and-a-half away.”

The Albosta committee’s final report stated the committee’s belief that there existed “organized efforts to obtain from the Carter administration, and from the Carter-Mondale campaign, information and materials that were not publicly available.” The investigators could not come to more precise conclusions, in part because the Carter White House — featuring many holdovers from the Ford and Nixon administrations who didn’t like the Carter gang — leaked like a sieve. The investigation found that 13 Reagan staffers had either received or were aware of Carter material that had come into the possession of the Reagan campaign. With so many possible culprits, Corbin escaped the noose.

At the height of the Albosta investigation, Baker received a call from Congressman Dick Cheney (R-Wyo.). Cheney told Baker that a member of his staff who had long known Corbin, Tim Wyngaard, confided that Corbin had privately acknowledged orchestrating the theft of the Carter briefing books and giving them to Casey. Wyngaard, the executive director of the House Republican Policy Committee, confirmed to Albosta committee investigators and to The New York Times that Corbin had claimed credit for lifting the briefing books.

At various other times, Corbin admitted directly or at least hinted that he’d stolen the briefing books. A number of sources have confirmed various aspects of Corbin’s role in the caper, including Cheney, John Seigenthaler and Bill Schulz. What’s more, Gerald Rafshoon, who was in charge of Carter’s media, recalled seeing Corbin around the Carter White House late in the 1980 campaign and thought it odd that this Kennedy man and Carter hater would be there. He had no idea at the time that Corbin was covertly working for Reagan.

Corbin did deny in a sworn statement to the Albosta committee that he’d given the briefing books to the Reagan campaign. But lying to federal officials was old-hat for Corbin. As Time magazine politely said, his “reputation for veracity is uneven.” Likewise, the FBI observed in one of its many reports on Corbin that he seemed to be a “prevaricator.”

Regardless, Corbin, the old master, left no fingerprints — in this case, literally. Although the FBI found both Jim Baker’s and David Gergen’s fingerprints on the briefing books, they found none of Corbin’s.

Corbin was too smart to make that dumb mistake. After all, how many Washington political operators had a downtown office with an unlisted phone number?