Four years ago Elmo R. (Bud) Zumwalt, his bushy gray eyebrows pointing into a brisk October wind, stood on a platform outside Alexandria's City Hall, pleading with his fellow Northern Virginians to support Jimmy Carter's bid for the presidency.

The efforts of the recently retired chief of naval operations were, as it turned out, about as successful as a liberty call in a dry port. Not only did Carter fail to carry the state, but Zumwalt's first bid for elective office, as the Democratic nominee to unseat Independennt Sen. Harry F. Byrd Jr., was crushed by Virginia's conservation electorate.

Despite Carter's promises of cuts in defense spending, Zumwalt stuck by Carter, continuing to dismay many of his old comrades in the Pentagon as he did in the days when his famed "Z-gram" orders helped make him one of the most controversial admirals in the Navy. When the president desperately needed support for his Panama Canal treaties two years ago, Zumwalt traveled around the country speaking for the administration.

This fall the 59-year-old retired admiral, now a Washington business consultant and newspaper columnist, returned to the political hustings. And once again he is leaving a trail of dismayed and astonished friends in his wake.

The reason: Zumwalt has abandoned his fellow Annapolis graduate Carter and many of the Virginia Democrats who championed his battle four years ago, to work -- as "a matter of principle" -- for Republican Ronald Reagan.

"I couldn't believe it when I heard he went to work for Reagan," fumed Virginia Democrat Henry E. Howell, the populist former lieutenant governor who campaigned for Zumwalt.

"If he didn't like Carter's [defense] policy he could say so, but he doesn't have to support Reagan who has said he thinks the minimum wage is the worst thing that's happened to this nation," said Howell, a longtime Carter supporter.

But the admiral charges that he was the one who was betrayed. "Carter said all the right things to me in private in 1976 and I thought he was going to be interested in correctinng our military weaknesses. I think it was just duplicity on his part," said Zumwalt.

Nor is the admiral any more impressed by the administration's claims that Reagan is a hawkish figure who would drive the nation into combat over minor foreign policy questions. "Of course that's not the issue at all but it's typical of the grungy, mean, ugly campaigns Carter runs," he said.

Zumwalt was interviewed in the Arlington office of Systems Planning Corp., one of the many defense contractors tucked into the high-rise buildings of Rosslyn, where he works as a consultant and writer earning $250,000 a year. o

His role for the Reagan forces didn't become well known in Virginia until he surfaced last month at a House Armed Services hearing and accused a National Security Council aide of leaking word of the Defense Department work on the Stealth bomber as a ploy to help Carter's reelection.

"The accusation is a lie," a White House spokesman fired back. "Admiral Zumwalt is either being misled or consciously playing gutter politics."

That flap pushed Zumwalt back into national prominence and focused attention on the independent group he heads -- Americans for an Effective Presidency -- that is supporting GOP presidential nominee Reagan. It also angered many prominent Virginians who said they worked hard for Zumwalt four years ago, knowing he had little chance of winning.

Perhaps few are as chagrined over Zumwalt's change as in Washington lawyer Timothy Finchem, who often struggled desperately as Zumwalt's campaign manager, seeking funds to keep afloat his low-budget race against the well-funded Byrd. But Finchem, now serving as national staff director for Carter's reelection, accepts the admiral's new course philosophically. "I suspect if Reagan were to be elected, Zumwalt might be considered for a position of some note in the administation," he said.

Zumwalt, who still sports one of the golden "Z" tie tacks that he gave to contributors to his 1976 campaign, disavows any interest in taking an appointive position in Reagan's administration and points out that he rejected similiar offers from Carter. He said that he doubts that as an avowed Democrat he would have a place in a Reagan administration. "Besides, I believe I would lose my wife if I went back to public life."

What impact Zumwalt will have on the Reagan campaign both nationally and in Virginia isn't clear. Republicans say his role illustrates the difficulty that ex-Navy man Carter will have in gaining support of the active and retired military personnel who may have supported him for four years ago. Official estimates place the number of active duty and retired personnel at 8 to 12 million nationally, about 500,000 of whom live in the Washington area, mostly in the Northern Virginia suburbs. In states such as Virginia where they comprise a significant segment of the electorate, they can be a key factor in elections.

Zumwalt says Reagan was not his first choice. He would have preferred John Connally or George Bush.

Virginia Democrats say his defection to Reagan is another reason why Zumwalt, who in Howell's words "parachuted" into the state four years ago when the badly divided Democratic party couldn't find anyone willing to take on Byrd, doesn't have a political future there.

"I would think he has a reputation as something of a loose cannon," said Larry Sabato, a University of Virginia government professor and the state's preeminent political analyst. "You wonder in retrospect how he managed to become CNO, because he's a born boat-rocker. There are great risks to be run in having a subordinate like Elmo Zumwalt."