One of the prime perks in Washington is having a government chauffeur who drives you to work. One of the prime perk-watchers in Washington is Sen. William Proxmire (D-Wis.), the relentless penny pincher who runs five miles daily to work.

He says that some of those being driven aren't entitled under federal law; they say they are, and complain that Proxmire is pestering them over a privilege one says "isn't any big deal." When Proxmire sent out letters this summer asking three dozen agencies and Cabinet departments to name the bureaucrats who get the service, he was met by some cooperation, but also confusion, defiance and silence. In fact, one of the great mysteries in Washington is still: Who does get driven to work?

"That's something we'd really like to find out," says Fred Fielding, the White House lawyer who's asked the Justice Department to give him a "reading" on the legal snarl. Ultimately, there'll be a resolution on who has to drive--and who gets the cozy back seat.

So all around town, some of Washington's most august public servants are scrambling to explain their ride in to work. They herald their long hours and low pay; their lawyers, meanwhile, are composing "Dear Senator" letters that defend the perk. But in an age of umemployment, budget cuts and a Beltway jammed with Toyotas, it's a formidable task marked by bureaucratic scuffles and political gore.

Welcome to Car Wars.

"One doesn't want to stick a senator in the ear with a sharp stick," says Defense spokesman Henry Catto, responding to Proxmire's complaint. "However, not everyone has the time or endurance to do what he does. I think it would be a grave mistake--and I'm saying this as a person who does not ride in a car to work--because these men do an enormous amount of work on their way to and fro."

"It's a great convenience," says Secretary of Defense Caspar Weinberger, one of the handful specifically entitled to the service. "I always read both ways."

The controversy stems from 31 USC 638a, the federal law that names the officials entitled to what has become known as "portal-to-portal" service. The law specifically mentions the president, principal diplomatic officials and Cabinet secretaries, but may or may not include their underlings, like deputies, who are driven to and fro with the permission of the boss. Proxmire, who's been after portal-to-portals since the mid-'70s, says the underlings aren't allowed; a comptroller general's opinion says, in essence, that they are. In practice, some underlings have had the privilege for many years.

A reporter's phone calls to the Cabinet turned up 12 "portal-to-portals" at Defense, five at State, four at Justice, three at Transportation and two at Treasury. Some selections: Deputy Secretary Frank Carlucci (Defense); retiring Deputy Secretary Walter Stoessel (State); Deputy Attorney General Edward Schmults (Justice); Deputy Secretary R.T. McNamar, who also drives to work himself (Treasury); and Adm. James Gracey, the commandant of the Coast Guard (Transportation).

Proxmire says each daily portal-to-portal service--including the time the official is chauffeured around town during the day--costs taxpayers more than $30,000 per year for both car and driver. (For instance, the Justice Department reports that car service for William French Smith, the attorney general, who is also specifically entitled to it, cost $52,206.70 in 1981.) Proxmire claims taxpayers spent $4.86 million in 1979 to drive 175 officials to and fro, a figure that includes scores of other officials at federal agencies as well. On Capitol Hill, both the Senate and House have appropriations bills that provide cars and drivers for the leadership.

But smart public servants have ready explanations for a perquisite they've grown fond of. Some cite their own departments' legal interpretations which say, in essence, that the official's car service is within the confines of the law. (The penalty for willfully violating it can be job suspension for up to a month without pay.) Others respond with:

* The Forthright Approach (Because My Time Is Your Money). "I car-pool with my vice-commandant," says Gracey, a four-star admiral. "It's an extra hour and a half of uninterrupted time when we can confer on issues about the Coast Guard. And we do. Incidentally, someone did a study in the Pentagon and figured out that a four-star admiral's time is worth $390 an hour. So each day, the taxpayers are getting $1,170 worth of work figuring in the comparable value of the vice-admiral for the price of that automobile."

* The Indignant Retort. "Frankly, I think that's dumb," says John White, who, as deputy director of the Office of Management and Budget under Jimmy Carter, said he used the service if he were working late or had to be at a work-related meeting first thing in the morning. "I moved from California to Washington, I took a substantial cut in pay, I worked for four years considerably under my market worth, I didn't accumulate any vacation, you don't get any retirement, and you work long hours. Right? So there happens to be this convenience where once in a while you get picked up and delivered to work. You're not riding in a chariot. How disagreeable do you want to make government service?"

* The Beat 'Em to the Punch. "Traditionally, the deputy director has had door-to-door pickup," says Joe Wright, the current deputy director of OMB. "But since we're reducing costs around here, I think this part of my job should probably follow the same path as stock options, bonuses and profit-sharing. Zero." Wright made his decision after OMB's lawyer decided--in response to Proxmire's letter--that Wright's service was "optional."

* The Forthright Approach II (Because They Need Me). "The rationale for me is twofold," says Rex Lee, the solicitor general, who uses the service during the October-to-June Supreme Court session. "One is because of the sensitive documents I handle during the session. It gives me an opportunity to work on them. And during that time, there's also a need for maintaining constant contact with the attorney general and deputy attorney general. There are phones in those cars."

* The Stickler's Reply. "Let me cite you the authority," says Capt. John Dewey, who is assistant to John Lehman, the secretary of the Navy, who gets driven to work each day. "There's a Department of Defense directive, number 4500.36, dated July 18, 1979. The subject is management, acquisition and use of motor vehicles. A paragraph in there says, pursuant to the exemption . . ."

"We've heard all kinds of excuses," says Proxmire, who claims that some of those include fear of earthquakes and having to be near a phone in case the president calls. "Democrats are just as bad as Republicans. They all love that limo." Back-Seat Blessing

That's because being driven to work is a truly wonderful experience. There's no traffic to fight, no noise, no strange rattle under the dashboard (at least none you have to worry about), no responsibility for gas, snow tires or inspection stickers, and much less of a chance to spill coffee in your lap. You don't have to park. You can read, you can make phone calls, you can stare out the window. Or, you can nap.

Certainly, portal-to-portal is considered a bauble of Washington power. Rock stars get it; so do corporate executives and foreign dignitaries. It says that your time is critical, that you're important enough to be carted around. It also impresses your neighbors.

Weinberger, who reads the newspaper and security briefings on the way to work, is a portal-to-portal enthusiast. He quotes John Gardner, the founder of Common Cause and a Cabinet member under Lyndon Johnson. "He said that 'the day you begin to enjoy having your car and driver is the day you should leave Washington.' Under that, we should all leave. Because I'm sure we all enjoy it."

Many of the driven, however, point out that the cars aren't that swanky. Although the secretary of state gets an armored Cadillac, Undersecretary of Defense Richard DeLauer says he has a "standard-sized" Ford. Most Cabinet secretaries get large cars such as the Oldsmobile 98 diesel, while deputies and undersecretaries get mid-sized cars such as the Oldsmobile Cutlass. Most of the door-to-door cars come from private executive leasing programs. They aren't part of the government's Interagency Motor Pool, the Washington fleet of almost 2,700 compacts, station wagons and other cars that various officials can check out during the day.

Nobody gets excited about those. As one undersecretary recalls about his first days in office: "I came over here, and I said, 'Where's my car and driver?' And they said, 'You don't get a car and driver.' So when I need a car, I get in a dirty station wagon with chicken bones in the back. It isn't right."

For those now out of government service, the memory of portal-to-portal lingers. "Oh, God, it was a blessing," recalls Brock Adams, the secretary of transportation under Jimmy Carter. "You read all the way, or you can call in and see if you've got any messages you can deal with on the way in. Then you hit the ground running."

A few do say they're not crazy about it. Ken Adelman, the deputy U.S. representative to the U.N., prefers riding his bike. "Really," he says. That's how he got to work when he was at the Pentagon. Now he shares his service, provided by the State Department, with three other U.N. staffers in New York. The driver picks up one person one block west of Central Park, then swings over to the East Side to get Adelman and two others. "No, you don't feel important," he sighs. "It's hard for anybody who works in the U.N. to feel important."

But the U.N. security people do worry about him. "They're always trying to talk us into taking different routes because they're afraid of lurking Libyans," Adelman says. "But we've never taken it too seriously. I always tell the driver that the terrorists will go for him first."

Occasionally, an official who normally doesn't get portal-to-portal will be given the service in unusual circumstances. Fred Fielding, for instance, was driven to work after another driver, who had picked him up at the airport, accidentally slammed the trunk on his hand. For several weeks, Fielding couldn't drive himself.

So how was it, riding to work? Fielding, a careful lawyer, responds: "It hurt." The Law

The law itself seems simple. It says, in part, that officials can't use government cars "between their domiciles and places of employment." The only notable exceptions to that law are cars "for official use of the president, the heads of the executive departments [the Cabinet] . . . ambassadors, ministers, charges d'affaires, and other principal diplomatic and consular officials."

"A literal interpretation would indicate that it could only be used for those individuals," says one administration official. So why do various undersecretaries say it's all right for them, too? "Because you have a lot of lawyers all over town who are trying to prove that the law doesn't mean what it says," says a Proxmire staffer.

Proxmire has been fighting this since the mid-'70s when, as chairman of the subcommittee on HUD and Independent Agencies, he fired the first shots of Car Wars. Annoyed by what he saw as a "caste system," Proxmire tried to pass legislation to get the cars taken from those he said were unentitled. But he couldn't get it approved. He also asked the comptroller general for an interpretation of the original law, but what he got was an opinion that said, in part: ". . . we have long held that use of a government vehicle does not violate the intent of the cited statute where such use is deemed to be in the interest of the government."

So Proxmire, whose committee at that time had jurisdiction over HUD and several independent agencies, decided to do what he could. He added a provision to an appropriations bill at least making it illegal for officials of the Veterans Administration, the Environmental Protection Agency, the National Science Foundation and other agencies to have the pickup service.

(That's why V.A. Administrator Robert P. Nimmo, who had previously enjoyed his back-seat ride to work, had to give it up. He also agreed to reimburse the government last month for the overtime wages of his chauffeur, $6,441.)

Fielding himself, emphasizing that he's not offering any legal opinion, does say that the phrase "for official use of" could be interpreted broadly. This, he says, justifies portal-to-portal for the White House "Big Three," aides Ed Meese, James Baker and Mike Deaver, as well as national security adviser William Clark, who also gets it for personal safety reasons.

Others who get the service, as reported by public affairs officials of their agencies or departments, include:

Gen. John Vessey, chairman of the Joint Chiefs of Staff; Gen. Charles Gabriel, the chief of staff of the Air Force; Gen. E.C. Meyer, the chief of staff of the Army; Gen. Robert Barrow, commandant of the Marine Corps; Adm. James Watkins, the chief of naval operations; Darrell Trent, the deputy secretary of transportation; and Warren Burger, the chief justice. The Bus Stops Here

It may be as hard to imagine Washington without double-parked chauffeurs as Washington without any perks. But every once in a while, a public official will temporarily forfeit his car to experience a less elite and more public form of transportation. Neil Goldschmidt, the second transportation secretary under Jimmy Carter, once took the bus to work. And former attorney general Griffin Bell, who says that the two Justice Department drivers "are the only link between all the attorney generals," once decided to leave the comforts of the car for a seat on the subway.

"But I was seized upon by the people," he says, "asking me what I was doing there. One man said, 'What are you doing on the Metro? You're supposed to be working."