Bill Diefenderfer, a former college wrestler and a brown belt in judo, sees similarities in his approach to combat sports and to the way he handles his job as chief of staff of the Senate Finance Committee.

You have to be single-minded, aggressive and -- Diefenderfer frankly admits -- a tough SOB on occasion to help negotiate the radical tax revision bill that passed the committee unanimously last week.

Committee Chairman Bob Packwood (R-Ore.) directed the dramatic turnaround that produced a tax-overhaul bill, but Diefenderfer -- whom Packwood calls "absolutely essential" to the success of the bill -- was the stage manager.

"If a brick wall appears, you have to go around it or through it or over it, but you have to make a decision," he says. "You've got to understand what your goal is, and everything that's irrelevant to your goal you push off to one side."

In some ways, William M. Diefenderfer, 41, is the quintessential congressional staff aide. He and Packwood are very close. Packwood's goals are Diefenderfer's goals. Diefenderfer left a lucrative partnership in a newly founded law firm to return to Packwood's employ, after having been chief counsel of the Senate Commerce Committee when Packwood was its chairman. He has worked for Packwood on and off since 1979.

"We read each other like mirrors, and it isn't very often that you come across something like that," Packwood said.

In other ways, the portly, bearded Diefenderfer stands out. He speaks for the record where aides generally talk to reporters without attribution. He sometimes negotiates directly with senators without going through Packwood. Although he has worked hard at learning the intricacies of the tax code, about which he knew little a year ago, he still leaves much of the detail to chief counsel John O. Colvin -- and to Packwood.

"If you gave the two of them an exam, Packwood would beat the hell out of Diefenderfer on who knows the most about the tax code," said Allen Moore, Commerce Committee staff chief and a former Finance Committee aide. "Diefenderfer's a strategist. He's not devoid of detail, but he's not as inclined as Senator Packwood to really delve into the details deeply."

The styles of the two men are similar. Packwood is known for saying what he thinks, for trying new ideas, for holding little back in public. As a result, he has been criticized for changing his mind frequently, but that quality also gives him the courage to try new approaches -- a tactic that paid off for the tax bill. Diefenderfer, who calls Packwood a friend, may have helped shape that characteristic.

"A lot of Packwood's boldness is Diefenderfer's boldness," a senior administration official said.

Diefenderfer calls Packwood "senator," not "Bob." But he is close enough to his employer to be able to treat him with something less than reverence.

Last December, for example, Packwood walked over to the House of Representatives to lobby members to pass his version of a fund to clean up toxic wastes. The House began voting on the bill just as he left, and Packwood's plan was going down miserably by the time he returned to the Senate Republican cloakroom, where senators and their aides were discussing the debacle. An awkward silence fell when Packwood entered, according to some who were there. It was broken only when Diefenderfer jovially hollered, "Good job, boss." Packwood laughed and the tension lifted.

It was during a conversation with Diefenderfer that Packwood had the idea three weeks ago to shock the then-moribund tax-writing process by introducing a radical plan to cut tax rates dramatically while wiping out numerous tax breaks for individuals and corporations.

That Friday morning, Packwood suddenly had pulled his earlier plan, bloated from the addition of special-interest breaks, off the table in order to avoid upcoming votes that would have hopelessly eroded the measure. Despondent, Packwood and Diefenderfer went to lunch at Kelly's Irish Times near the Senate office buildings.

Some time during the second pitcher of beer, Packwood raised an idea: Why not junk the whole thing and come up with a radical tax-reform plan that would cut the top rate in half, to 25 percent?

"He looked at me and I looked at him and I said, 'Why not?' " Diefenderfer recalled. And they went back to the office and asked the staff of the Joint Committee on Taxation to draw up such a plan.

"If we had had three pitchers, the top rate would have been 10 percent," Diefenderfer joked.

The rest is history. After a week of private negotiation, in which such popular deductions as the one for mortgage interest were restored, senators supporting the bill got a few concessions and the top rate was changed to 27 percent. The plan passed the committee just after midnight last Tuesday by a 20-0 vote.

"It is the most remarkable legislative accomplishment at least to this point [that] I have seen," said Sen. John C. Danforth (R-Mo.). "They're pitching a no-hitter game here. It took a lot of daring and a lot of imagination. Bill has a real feel for bringing diverse people together."

Not bad for a staffer who early in his tenure was the butt of "amateur-hour" jokes. Diefenderfer, several sources said, has matured considerably in his year and a half at Finance.

"The metamorphosis of Bill Diefenderfer as chief of staff runs almost parallel to the tax-reform process," said an aide to one Finance Committee senator. "He felt you could deal with tax bills the way you did at Commerce, that they were just a big puzzle to be put together, that you could do one thing without changing the total picture. Then he got an appreciation that it has to be a consensus process."

During the final negotiations over the bill, congressional sources said, Diefenderfer probed for what senators could live with. Those who were involved in putting together the new plan got a few home-state breaks retained. Those who were outside the process often found their friends -- such as the oil refiner constituents of Sen. David L. Boren (D-Okla.) -- paying more taxes.

Packwood made a point of saying he, not his staff, had made the decisions on who got what. But Diefenderfer himself says his job requires him to be hard-nosed at times.

"I am combative, if the chairman has set a goal for me," he said.

Talkative, too, sometimes. Diefenderfer occasionally raised the hackles of senators on the Finance panel when the first tax plan was being written by freely discussing its details before well-heeled business groups, but revealing little to committee members or reporters.

"There are times when he should have kept his mouth shut," one congressional source said.

He has brought many former Commerce aides to the Finance Committee, mostly to replace aides who have left to join the private sector. He fired the press secretary, but says budget constraints forced him to do it.

Diefenderfer, a lawyer and a Pennsylvania native, served in Vietnam after graduating from Dickinson College, and earned a law degree at Duquesne University. In Washington, he worked for the House Education and Labor Committee, the Ford White House (on the domestic policy staff) and former representative Bruce F. Caputo (R-N.Y.) before moving to the Commerce Committee.

He helped found a law firm after leaving Commerce. "I worked off a coffee table for three months before we had an office," he recalls. But he sold his partnership when he went back to work for Packwood.

Diefenderfer will leave the committee, he says, "soon after the president signs the tax bill." Although chances for tax revision have improved significantly with the turnaround and subsequent unanimous committee passage, it still faces opposition on the Senate floor from outraged lobbies and hostile senators. Consideration is expected to begin next month.

Diefenderfer says gaining knowledge in the tax area has "opened up some new vistas for me" in terms of future employment. Asked if he would feel any embarrassment at joining the phalanxes of former Hill aides lobbying on tax legislation, he shrugs and says, no, "There are good lobbyists and bad lobbyists and a lot of in-between folks."