Jonathan R. Steinberg is one of this city's "shadow players."

His name rarely appears in print and he is not widely known outside Capitol Hill, where he is Democratic chief counsel to the Senate Veterans' Affairs Committee and a key aide to its ranking minority member, Sen. Alan Cranston (D-Calif.).

But in the past 16 years, Steinberg has written hundreds of pages of legislation, most of it dealing with veterans. It would be difficult to find a recent law that affects veterans that Steinberg did not help shape.

He was the principal author of a controversial 1970 law that extended free Veterans Administration medical benefits to all veterans once they reach 65. He drafted the law that created storefront counseling centers for Vietnam veterans and wrote legislation that forced the VA to treat veterans who suffered health problems after they were exposed to radiation during atomic tests.

All members of Congress rely on close assistants to help turn their ideas and political promises into laws, but Steinberg, 45, has a reputation for being unusually skilled at those tasks.

"He's simply awesome," said Max Cleland, a friend of Steinberg's who served as VA administrator under President Jimmy Carter. "I don't think there is another single individual who knows more about veterans' legislation, the process, the politics, and is more dedicated to his boss than Jon."

Steinberg first became interested in politics and law at Cornell University, where he said he spent more time participating in student government than studying. He later graduated with honors from the University of Pennsylvania Law School, where he was an editor of the law review.

After graduation, he came to Washington to work as a law clerk for Warren E. Burger, then a federal appellate judge. He ignored Burger's advice in 1964 when he turned down an offer from the Justice Department to become an attorney for the Peace Corps.

In 1969, a friend introduced Steinberg to Cranston, who hired him as his chief adviser on human services issues.

At the time, Cranston was a freshman senator who was under attack from conservative Californians and the state's defense contractors because he opposed the Vietnam war.

Partly to appease those opponents, Cranston successfully sought the chairmanship of what was then the rather unimportant Senate veterans' affairs subcommittee. When the Senate created a standing committee for veterans' issues two years later, Cranston found himself a step ahead of his colleagues and a rising star among veterans' groups.

"We don't agree with most of Sen. Cranston's stands on national issues," explained Robert E. Lyngh, veterans' affairs director of the 2.5 million-member American Legion. "But I tell our members, 'Look, this guy is a strong veterans' advocate who really watches out for us.' "

Cranston's and Steinberg's clout increased dramatically under Carter, who chose Steinberg to head his administration's transition team at the VA, giving him the opportunity to help set that agency's agenda. Carter also hired Cleland based on Cranston's recommendation.

Along with his chairmanship of the Veterans' Affairs Committee, Cranston became majority whip in 1977. As a result, Steinberg took over most of the responsibility for conducting committee business.

During this period, Congress enacted legislation that Steinberg drafted and Cranston introduced that expanded veterans' benefits and the VA hospital network.

After watching the two at committee hearings, Fortune magazine in 1979 reported: "It becomes clear why lobbyists for veterans' organizations refer to the Californian as Senator Cranberg. Steinberg's whispered words are often repeated by Cranston as if he were a ventriloquist's dummy."

Under the Reagan administration, Cranston and Steinberg have spent much of their time protecting VA programs from budget-cutters and getting the agency to enforce laws that the administration opposes.

Steinberg has critics, including some at the VA, where he has been known to trample toes. But the worst thing that most critics say about him is that he is tenacious or too pushy. Some veterans also view him with suspicion because he never served in the military.

"Not being a veteran was an impediment, particularly in the early years," said Steinberg. "But after 16 years, I think I have overcome that."

There are signs that he still is uncomfortable about that, however. Unlike his committee colleagues who are veterans, Steinberg did not attend the recent unveiling of the statue at the Vietnam Veterans' Memorial because, he said, he would have felt like an intruder.

His friends call that ironic. "Few people have done as much as Jon for veterans," said Cleland, "particularly Vietnam veterans.