At noon Tuesday, Arlington County Board Member John G. Milliken formally filed his candidacy as a Democrat seeking Republican Rep. Frank R. Wolf's seat in Congress.

Milliken and a number of 10th District Democrats are saying that Milliken should have the best chance of any challenger to beat Wolf since Wolf was elected in the 1980 Reagan landslide. They have 222 days before Election Day Nov. 4 to prove that they are right.

Milliken, a 40-year-old Washington lawyer, has his work cut out for him. In Wolf, Milliken faces a conscientious incumbent with few negatives.

Now in his sixth year in Congress, Wolf, 47, has managed to identify himself with the interests of federal workers while not straying too far from the policies of the Reagan presidency. On transportation, a cornerstone issue for many of his constituents, he has been both vocal and visible.

Since defeating Democrat Joseph L. Fisher in 1980, Wolf has not faced a challenger rated as a good bet to beat him.

In 1982, he soundly defeated former Arlington state delegate Ira M. Lechner, who was plagued by disarray within his campaign and a left-leaning image in a right-leaning district. Wolf received 54 percent of the vote, beating Lechner by more than 12,000 votes.

In 1984, Wolf improved on his margin with a one-sided victory over John Flannery, a former federal prosecutor whose Kennedy good looks failed to make up for an overblown rhetorical style and virtually no track record on local issues. Flannery's purported "coalition of dreams and pain" failed to materialize, and Wolf trounced him by 58,000 votes out of 240,000 votes cast, taking 62 percent.

Despite those two wins, Wolf's seat is a long way from being safe. The 10th District is widely perceived as neither solidly Republican nor Democratic. Since the early 1970s, it has turned from Republican to Democratic and back to Republican in congressional elections.

Democrats think that Milliken, a former aide to Fisher, is their best chance to move the district back to the Democratic column. Their growing confidence is based on these factors:Unlike Flannery, who had no record in office, and Lechner, who had failed in two previous bids for the party nomination for lieutenant governor, Milliken is a proven vote-getter with a solid base in Arlington.

In his bid for a second term at-large on the Arlington County Board in 1984, he received more than 70 percent of the vote -- one of the highest totals a candidate for any office has received in the county.

* Milliken has two important allies that Wolf will be hard-pressed to match. One is former governor Charles S. Robb of McLean, Milliken's honorary campaign chairman, who is likely to make appearances and raise money for the campaign. Milliken touts Robb, whose fiscal conservatism and social progressiveness earned him popularity throughout the state, as his ideological kin.

Party regulars also say Milliken's personable wife Chris will be an invaluable asset in the fall campaign.

* Northern Virginia Democrats say they have a pronounced edge over the GOP in political momentum, coming off resounding victories statewide and in the 10th District last November by Democratic Gov. Gerald L. Baliles, Lt. Gov. L. Douglas Wilder and Attorney General Mary Sue Terry.

"Our party's just as high as a kite," said Fairfax County Democratic Party Chairman Harris N. Miller.

By contrast, Fairfax County Republicans are in the midst of a bitter, divisive battle over the party chairmanship, with former sheriff James M. Swinson challenging conservative chairman Benton K. Partin.

* The Democrats' soaring morale, which they hope will translate into an organizational edge in the fall, could be all the more important in this off-year contest in which turnout is expected to be low.

The lower the turnout, strategists on both sides say, the more important the party machine.

Moreover, the race in the 10th District this fall will probably occupy the political spotlight in Northern Virginia. In the 10th District, portions of Fairfax and Loudoun counties, it will be the only race to appear on the ballot.

In the 8th District, Republican Rep. Stan Parris is unlikely to face a serious challenge in his bid for reelection, even if former Alexandria mayor Charles E. Beatley Jr. gives up his retirement to launch a long-shot campaign for Congress.

Republicans say publicly that they are confident about Wolf's chances for a fourth term, but privately they say that they are running scared. One measure of their anxiety is that Wolf has already raised about $200,000 for the race, which is still more than seven months away. Wolf aides won't name a target, but say they are confident of raising "significantly more" than the $650,000 he spent in walloping Flannery in 1984.

Milliken won't say how much he has in hand, but he says he needs at least $550,000 to get his message across in the fall.

To win, Democrats calculate, they will need a resounding victory in Milliken's home territory of Arlington, while still managing to take a respectable 45 percent or so of the vote in Fairfax County's portion of the district. Loudoun County, which accounts for about 12 percent of the electorate in the 10th District, is traditionally strong ground for Wolf, but could be attracted to Milliken, who grew up there.

Wolf will probably try to neutralize Milliken's advantage in Arlington and pile up a big lead in Fairfax, where he won by a 2-to-1 margin in 1984 and has good name recognition.

"I think it's the most serious race in the district in a long time," said Milliken.