WASHINGTON - The archivists at the Library of Congress know well the ruddy face and tenacious mind of researcher Michael Hill.

He might arrive looking for the U.S. Senate records of Harry Truman, or a newspaper clipping about boxer Jack Johnson, or background on Gen. George Armstrong Custer. He might show up alone, or with a famous historian, perhaps David McCullough or Ken Burns.

If there were a category for "popular researchers," the list would have to begin with Hill, who can be found in the credits of so many books and documentaries, from McCullough's "John Adams" to Nathaniel Philbrick's "Mayflower" to Burns' baseball series. The general public may look past his name, but historians regard him as almost a second mind and body, the one who finds the fact no one else can come up with, the one who not only assists on a project, but at times helps change its direction.

"I love him like a brother," Burns says. "He's super smart and super modest and super diligent. If you want to be in a foxhole with anybody, it's Mike Hill."

His gifts for assisting others helped place Hill's name on the cover of a book this fall. "Elihu Washburne" is a collection of private journals and correspondence by the 19th century politician and diplomat. The book was edited and annotated by Hill, and McCullough wrote a foreword, in part an expression of gratitude for what was an essential part of the historian's best-selling "The Greater Journey," which tells of American artists in Paris in the 19th century.

McCullough asked Hill to look through the papers of Washburne, the American minister to France during the Franco-Prussian War and an eyewitness to the siege of Paris in the early 1870s. The Washburne archive at the Library of Congress did not mention a diary, but an addendum referred to a diary at the Washburne homestead in Livermore, Maine. A trip there and hours of searching turned up a box with the original diary.

The New York Times' Janet Maslin gave "The Greater Journey" a mixed review in 2011, but cited the diary as the "book's single best research discovery."

Research itself was an unexpected passion for Hill, as if stumbled upon in someone's basement. Like McCullough and Burns and others he works for, Hill is not an academic and received no formal training for his profession. A native of Honesdale, Pa., he was a political science major at Kent State University, received a law degree at the University of Akron and a master's in public administration from Harvard's John F. Kennedy School of Government. After college, Hill was a press aide for then-Vice President Walter Mondale and later for then-Rep. Robert Torricelli of New Jersey.

But by 1982, he was in personal and professional crisis, unhappy with politics and in a marriage on the verge of collapse. Around this time, Hill read McCullough's classic work on the Brooklyn Bridge, "The Great Bridge," and was so engaged that he decided to write to the author and volunteer his help. He learned that McCullough lived on Martha's Vineyard in Massachusetts and reasoned that he might appreciate having someone in Washington to look up documents. His letter was addressed "David McCullough/Martha's Vineyard."

"A couple of months went by and I totally forgot about it," Hill says. "I came home one day, a particularly beaten down day, opened up the mailbox and there's this letter from David. He was moving to Washington to work on the Truman book. ... I called him the next day, and he said he had research assistants before but they hadn't worked out too well. `Since you're here and I'm here, let's have lunch.'"

Word spread about his good work. McCullough introduced Hill to a young documentary maker named Ken Burns, then in the middle of a film about the late Louisiana populist Huey Long and soon to begin his Civil War documentary. He jokes about having to share Hill's time with McCullough.

"You could tell it was a dutiful decision on David's part whether he would let me know about this extraordinarily resourceful human being," Burns explains with a laugh "He (McCullough) just said, `If you need a blue Volkswagen turned upside down on a downtown street tomorrow morning, he'll figure out a way to do it.'"

Historians praise Hill not just for the obvious gifts of knowledge and agility, but as one who pushes just a little harder than other researchers and gets archivists to do the same for him. Jeffrey Flannery, who heads Reference and Reading Services for the Library of Congress' manuscript division, has known Hill for years.

"Any archivist gets some deep satisfaction to see the collections used," Flannery says. "And Mike is a vital cog in that. You help him as he does his work and a couple of years later there's a book using our materials."

A resident of Frederickburg, Va., Hill spent a recent afternoon at the manuscript division, looking through a bound Washburne journal that was perched on a foam rubber easel. But he also has traveled with McCullough from Paris and Maine to the Truman library in Independence, Mo. He visited England to help Michael Korda research a biography of T.E. Lawrence. Philbrick was glad to have Hill along for "The Last Stand," about the Battle of the Little Big Horn.

"I wanted to retrace Custer's steps, 300 miles over sparsely populated terrain," Philbrick says. "We were out there on the terrain and got stuck in the mud in the place where Sitting Bull was killed. We've had all sorts of adventures and having him there was an immense help."

Like a beloved supporting actor, Hill need only worry about keeping up with his commitments. McCullough has him lined up for a second book on Americans in Paris, this time moving ahead to the 20th century and the impact of air travel. He's helping Philbrick on a Revolutionary War book, Korda on a biography of Robert E. Lee and former Mondale chief of staff Richard Moe on a study of Franklin Roosevelt. Hill is even attempting another book of his own, about World War I poet Alan Seeger, best known for "I Have a Rendezvous With Death."

"I'm drawn to his story," he says of Seeger, the uncle of musician Pete Seeger. "There have been some small, but not particularly good bios of him long ago. There is new archival material on him. And the centennial of World War I is coming up in a few years.

Hill adds: "It will take me a long time to do it."