The words kept coming to the tip of Frank Wolf's tongue, but the three-term Republican congressman gingerly sidestepped a formal reelection announcement, preferring instead to join in the revelry of the 47th birthday bash in his honor.

But any doubts that his announcement is forthcoming were quickly dispelled by his thinly veiled words to 400 jubilant supporters who had plunked down at least $15 each to attend the birthday party at a Tysons Corner hotel last Thursday that added about $34,000 to Wolf's $104,000 campaign chest.

"This isn't an announcement," said Wolf, who represents Northern Virginia's 10th Congressional District. "But I want you to know I have the same birthday as FDR, and I hope to be able to stay around as long as FDR."

The crowd roared approvingly, and joined in as Sen. Paul S. Trible (R-Va.), who predicted Wolf's "resounding reelection victory" this fall, led a chorus of "Happy Birthday."

There were letters of congratulation from President Reagan and his wife Nancy and from Sen. John W. Warner (R-Va.), who pledged his "solid commitment" to Wolf's reelection.

Amid the cakes, cheese trays and vegetable platters, there also were reams of nominating petitions for Wolf's supporters to circulate to assure his placement on the ballot for the 10th District, which covers Arlington, Loudoun and northern Fairfax counties and the cities of Falls Church and Fairfax City.

As a band and a barbershop quartet entertained in the hotel ballroom festooned with red, white and blue balloons, Wolf supporters outside sold red drinking cups, bumper stickers and sun hats bearing Wolf's name for $1 each. At a nearby table, other volunteers gave out name stickers that were color-coded according to the size of the individual guest's contributions: benefactors ($50), sponsors ($100) or patrons ($250).

Wolf said in an interview that he will not announce his reelection bid for a "couple of months." But it was clear that he was gearing up for the race, taking nothing for granted because of what many of his supporters note could be his toughest campaign since unseating former Democratic Rep. Joseph L. Fisher of Arlington in 1980.

It will be a campaign that, by all accounts, will match him against Democrat John G. Milliken, a popular Arlington County Board member and former Fisher congressional aide who is expected to announce his candidacy soon.

Asked if he is expecting a challenge from Milliken and how difficult that could make the campaign, Wolf said: "I don't know. I don't know what John is thinking . . . . He's an okay guy . . . but I intend to run a very positive campaign . . . . I think we're just going to take my record to the people. I think I've done a lot for them."

He then changed the subject to races of another sort, such as his participation in the Reston triathlon last September and the Boston Marathon several years ago. In the ballroom, however, his supporters talked about the upcoming political race, which will be the only one on the ballot in Fairfax and Loudoun counties.

For this reason, several Republicans said, they will have to work hard to get out those counties'traditionally Republican votes to offset the usually large Democratic vote in Arlington, which will also have its annual County Board election and, perhaps, a race for county prosecutor.

Although Wolf captured Arlington for the first time in his 1984 reelection to Congress, winning 53 percent of the vote there, Milliken amassed an unprecedented 70 percent of the vote in his successful bid for reelection to the County Board that year, winning in the county's solidly Republican precincts.

But the Loudoun-born Milliken is untested in the more populous areas of the 10th District, where Wolf has won with increasingly higher tallies, collecting 63 percent of the district's vote in 1984 against 51 percent in his 1980 campaign against Fisher.

"You can't take Wolf's reelection for granted," said Betsy Nutt, one of his supporters from Fairfax County. She said that Milliken "is a man who can sneak up on you. He's a good vote-getter in Arlington. If Arlington Democrats get their vote out, it would be a little closer.

"I don't want to say it's going to be a tough race, but I don't want to be complacent about it," Nutt said. "It would be a closer race than we've had for a while."

"I don't think it will be quite as tough as Wolf's race against Joe Fisher, but it will be real tough," said David Burgess, an Arlington resident. "Fisher had a longer track record in Northern Virginia politics than Milliken, and Milliken is his protegee. But Frank will still win."

Joanne Alpern, an Arlington lawyer who has served on Wolf's advisory committee on family issues, concurred: "I think it will be a good race because Milliken has experience in public service.

"But Frank has done so much for the district," Alpern said, that Wolf should win reelection handily.