Just hours after he won landslide reelection to the Arlington County Board, Democrat John G. Milliken stood beside his political mentor, Joseph L. Fisher, at a victory party and listened to the former Northern Virginia congressman's words of congratulations.

After extolling Milliken's record during his first four-year board term, Fisher, a former member of the County Board, turned to the cheering throng gathered in a church hall.

"The vote was an indication of the way people feel about the kind of job John has done and could continue to do on the Arlington County Board," said Fisher.

Then Fisher grinned and said of his former congressional aide's political prospects: "Or on something else."

"Something else. Something else," the jubilant Democrats began chanting.

To many Northern Virginia Democrats, Milliken is Fisher's heir apparent, the best candidate the downtrodden party may have in 1986 to oust Republican Rep. Frank R. Wolf from his 10th District seat.

Democrats remember, with some bitterness, that it was Wolf who turned Fisher out of office in the 1980 Reagan landslide and who defeated Democrat John Flannery this year by what is believed to be the largest margin ever -- 62 percent of the votes cast -- in the 10th Congressional District.

Still, Democrats' hopes were buoyed by Milliken's overwhelming defeat of Republican-backed independent Peter Espada: a 70-to-30 percent margin in which Milliken captured nearly 42,000 votes, believed to be the largest tally ever for any candidate on an Arlington ballot. The vote, Arlington Democrats gleefully pointed out, shattered the record of 39,500 votes set by President Richard Nixon in 1972.

The only Northern Virginia Democrat to win a contested race this year, Milliken was the first Arlington County Board candidate to capture all 39 of Arlington's precincts, amassing impressive victories in even traditionally bedrock Republican precincts.

That, his supporters say, underscores Milliken's ability to appeal to voters of all political stripes, a coalition crucially needed to establish a viable base in the 10th District which includes Arlington, Loudoun and northern Fairfax counties and the cities of Falls Church and Fairfax City.

The supporters also note Milliken's ties to Loudoun, where he grew up, his contacts statewide as secretary of the state Democratic Party and his familiarity in the district from the Fisher era and as a public official.

But most Republicans and a few Democrats say the County Board contest was not a real litmus test because Milliken's opponent was so weak.

Although Milliken is very popular in Arlington, they add, he is untested in Fairfax and Loudoun, where a negative vote can cancel out any significant victory in traditionally liberal, but much smaller Arlington.

"I think John might find some real problems and challenges in Fairfax and Loudoun, where there is a greater population," said Robert R. Bocek, who is chairman of the 10th District Republican Committee.

Although Walter F. Mondale took Arlington this year, Republicans are quick to note that Wolf and Sen. John W. Warner also won there for the first time and with substantial margins.

Milliken, who has earned a reputation as a moderate and fiscal conservative on both the County Board and as chairman of the Metro transit board this year, said: "It's way too early for me or anybody to think about a 1986 race." And too early, he acknowledged, to rule out one.

Instead, Milliken, a 39-year-old lawyer with Mondale's Washington firm, is interpreting his victory as Arlington voters' reaffirmation of their confidence in the County Board's policies.

"Whether he's actually going to be a contender or not, I think his victory makes him so in people's minds," said Del. Warren G. Stambaugh, another Arlington Democrat. "Anybody who gets that kind of incredible margin has got to be put in the category of frontrunner."

"He has stronger appeal than all the other elected Democrats in the 10th District to the independents and Republicans, and to keep the Democrats together," said Arlington's Democratic Sheriff James A. Gondles. "He probably doesn't want to read about this, but, other than Joe Fisher running again, John Milliken is the Democrats' best hope of getting that seat back . . . .

"I think Frank Wolf hears the footsteps of John Milliken right behind him."

"Frank doesn't want to speculate on what John Milliken might do in 1986," said Jim Boyle, a Wolf spokesman. "From our perspective, John Milliken certainly had a very impressive victory. But Frank Wolf's was just as impressive."

So impressive, Boyle added, that Wolf, now heading into his third term, "does not feel bound" by his previous pledge to retire after six terms, and will take it "one term at a time." Wolf's victory margin was so substantial, Boyle said, that he doubts any Democrat will be able to get much financial support from groups targeting Republican congressmen in 1986.

While Republicans and many Democrats expect Flannery to make another bid for the Democratic nomination in 1986, Republican John F. Herrity, chairman of the Fairfax County Board of Supervisors, says Milliken would be "a lot stronger" than Flannery.

But, Herrity adds, Milliken "is going to get whipped if he runs for Congress." Herrity said he doesn't believe Milliken is well enough known in Fairfax and is too liberal.

"Frank Wolf's base, which is Fairfax, is pretty strong. Milliken has a base in Arlington. It's rather obvious the battleground's going to be Loudoun," Herrity said. "If you look at it, I would think twice about running against Frank Wolf."