Alexandria Vice Mayor James P. Moran donned his running shorts and English-made racing shoes Thanksgiving Day and ran through the streets of Potomac West in the neighborhood's annual 5-mile Turkey Trot. Boston-born Moran, an ex-football player, said he did it for the exercise. "It seemed a more constructive use of time than pigging out."

Equally important perhaps was the fact that the race was sponsored by a group of merchants with whom Moran frequently had been at odds. But their contentious, sometimes rancorous relationship seemed a thing of past on Thursday. Moran not only ran five miles, he handed out their prizes afterward.

Constructive use of time, indeed. Just the way the 37-year-old Moran has operated most of his meteoric political career in Alexandria. That career began in 1979 when he left a powerful Senate staff position on Capitol Hill to devote himself to his newly won, part-time City Council seat in Alexandria. He became vice mayor, the city's No. 2 job, last May, after polling more votes than anyone ever has in a city election.

And now the wisdom is that Moran is a cinch for the mayor's job in '85 if current Mayor Charles E. Beatley retires. Moran, now an Old Town stockbroker, has hinted broadly that that job interests him, but so, too, does the seat of Rep. Stanford E. Parris, the Republican who represents Alexandria, eastern Prince William and southern Fairfax County.

With his personable campaign style, energy, intelligence and ambition, Moran has managed to established himself as a political power in Alexandria in barely five years. "He's a very sharp individual," says Beatley, a Democrat regarded by some as Moran's mentor. "He was ready to be mayor this time; he could have run," Beatley said. "He wasn't a protege of mine, he's too strongwilled for that."

The mayor fears, however, that Moran may be attempting to move too rapidly, a position other Northern Virginia Democrats share. "I've heard people say that he's going too fast," says retired Alexandria councilman Nelson Greene, another Moran ally. "He'd give anybody a good run for the mayor's job, but I think he'd be spinning his wheels if he tried for Congress in '84."

What Moran's supporters and detractors agree on is the effectiveness of his Irish charm. Less garrulous than a good listener, with a voice he uses like a musical instrument, roaring at a joke, dropping low the next minute with concern, Moran slaps backs and busses babies with the elan of the New England politicians he grew up among in Natick, a Boston suburb.

After a boyhood in a neighborhood that boasted former House speaker John McCormick and current Speaker Thomas P. O'Neill, Moran went to Holy Cross College on a football scholarship. After a brief stint as a Wall Street stockbroker and graduate student, Moran arrived in Washington, landing jobs in high places, establishing himself as protege of one boss, then another, first at the old Department of Health, Education and Welfare then the Library of Congress, then on the Senate Appropriations Committee. "The moves I made all turned out to be propitious," Moran says.

So too was his civic activism in Alexandria where he settled with his wife and two children. That marriage ended in 1975, but Moran had become active in social service groups, and four years later he handily won a seat on Alexandria's seven-member City Council his first time out.

He left Capitol Hill the same year because, he said, "I didn't want to become a sycophant. I liked being on the City Council, making decisions on my own."

Others who know him say the decision to leave the Senate was more calculated. "He made the critical decision to leave a very fine job in the Senate where he could have become staff director of the committee, or eventually an assistant secretary. Instead, he plunked down in the middle of Alexandria and began mentally mapping out a strategy," says Robert Leider, a local publisher who has worked in Moran's campaigns.

"Jimmy has the fire in the belly to endure the slings and arrows of public life in a discontented society like ours," says Bill Perkins, a longtime Alexandria Democrat. "You have to want it bad. He does. He's a creature of the Congress; he wants to go back. I've been around Washington for 40 years and I take it as gospel that you don't work up there for all those years without wanting into that neat little club."

Ambitions or no, Moran made his name in Alexandria with work for social service groups such as the United Way and lobbying for sheltered homes for mentally retarded. Moran has built a constituency among the elderly, indigent and minority communities in Alexandria, and they have proved loyal. "I think he could be president," says one.

"Jimmy's big thing was social services," says Perkins. "I don't think he did that solely, or even mostly, for political reasons. In one half of his . . . nature, I'm sure Jimmy went to sleep crying about the plight of the homeless. The fact that that also happens to be a good springboard to local politics is something the other half knew."

Through his work on social service cuts, on tenant rights, housing issues, Moran, perhaps more than any other Alexandria official, has become a symbol of concerned idealism. "He just makes us feel at ease, he makes us feel human," says Thomas E. Taylor, an Alexandria senior citizen who worked for Moran.

Others say Moran has moderated since his first term, has become more of a fiscal moderate, and is more likely to examine all sides of an issue than he was during his first term. Moran, who earns $12,500 a year in his parttime city council job, has harped on high city salaries, and publicly berated a department head whose failure to spend $90,000 in federal funds this spring meant the money would have to be turned back to the federal government.

"He doesn't hide what he is. He belongs to a different breed of Democrat," says former Republican Vice Mayor Robert Calhoun. "Like a lot of new Democrats, he realizes the days of wine and roses are over."

Calhoun says many Alexandria Republicans don't share that view of Moran and would be delighted if he challenged Parris in 1984. "I'd love see him run; he'd get his ears boxed," said former Republican Del. David Speck, another Alexandria stockbroker.

"If I ran for Congress in '84 I could still run for mayor in '85," says Moran. "I'd like to be in congress. I know those issues, I worked there. I'd be good at it."

It may seem early to be bidding for the seat, but the jockeying has begun. Moran has asked for a January audience with Tip O'Neill, a friend of his father's. He is meeting with the executive director of Pamela Harriman's political action fundraising committee for the Democrats. His brother works for Rep. Barney Frank (D-Mass.), and would help him plan a campaign if called.

"I'd like the job, I was there in Congress enough to know. I like the issues -- taxes, finance, those are the things I know something about. And the other issues, like nuclear and the third world."

Whether the next two years will give Moran enough time to break out of the sometimes insular world of Alexandria politics and establish himself in Fairfax County, the larger, more populous section of the congressional district, may be a more immediate challenge. "Who's Jim Moran?" says the county's senior Democrat, State Sen. Adelard Brault, when told of Moran's ambitions.

"I don't know the gentleman," says Brault, "That's a fact. But if he's thinking about running in '84, he should be busy right now getting some name recognition in Fairfax County. I wouldn't rule it out, but for anyone from Alexandria, it's going to be hard."