BOSTON -- If you want the magic formula for the Incredible Expanding Pack of presidential candidates, seek no further. The mystery comes in a three-word package: Why Not Me?

Without a front-runner, without a giant-slayer, without a bigfoot in the race, a parcel of would-be contenders are still asking themselves what the other guys have that's so special. Enter Pat Schroeder. Or to be more accurate, maybe enter Pat Schroeder.

The senior woman in the House of Representatives, former cochair of the Hart campaign, has just announced that she is going to ''look seriously'' at the prospects of running for the Democratic presidential nomination. Indeed, this Coloradan has already ''looked seriously'' at the competition.

At 46, Schroeder is about the mean age of the men. With eight terms in Congress, mostly as ''Czarina'' of the House Armed Services Committee, a host of foreign experience and a squeaky-clean reputation, she's on the long side of qualifications.

Ann Lewis, longtime Democratic adviser, ranks her this way: ''Schroeder's passed more legislation than Joe Biden. She's been in Congress longer than Gephardt. She's got more personal connections in Iowa than any of them. I could add that she's got a better haircut than Al Gore, but that would sound tacky.''

Nevertheless, by way of telephone greeting, Schroeder asks: ''What do you think? Have I lost my mind?'' Qualifications on paper don't always translate into votes, especially for women, and she is the only woman to come this close to actually running in '88.

Schroeder describes herself as frustrated by what she's heard. Other candidates haven't given top priority to the two issues for which she holds her strongest credentials: family policy and arms control. Every man has tipped his hat to ''family,'' but she has led the fight in Congress, especially for the parental-leave bill. Every candidate is for world peace and arms control, but Schroeder took the lead in Congress for the nuclear test ban.

''Why sit around and wring your hands waiting for the other candidates to talk about these things. I've always said that women have to run just like men do. After Hart dropped out, people began to call me and say: Why not just do it?''

The ''Why Not Me'' formula makes some sense. Schroeder begins with support and name recognition among women activists. Just a few days before the Hart debacle, a poll showed that 75 percent of the undecided voters in Iowa were female. However you analyze that figure, it suggests that a large pool of women are less than enthusiastic about the available men.

Says Ann Lewis, who's been a Jackson adviser, ''There is a yearning out there for a candidate who takes women seriously, and that is best represented by a woman candidate. There's a strong feeling that in their zeal to connect with white male voters the candidates are ignoring women.''

Yet there is also ambivalence about a potential Schroeder candidacy. The common wisdom even among political women has been that this is ''not the year for a woman.'' According to the long-term game plan of women strategists -- the A Team -- this was the post-Ferraro time for regrouping and planning. But as Lewis says archly, ''Being taken by surprise in politics is a gender-free experience.''

Says one activist bluntly and off-the-record: ''We're not interested in seeing her run and do poorly.'' ''If it backfires and she looks foolish, I would hate to lose her in Congress,'' says a second worrier. Still others wonder whether it's already too late, in terms of organizing and supporters: ''We see the positives in terms of her vision and the negatives in terms of the details of running a campaign. ''

As for the crucial ingredient for any rising campaign -- money -- Ellen Malcolm, the head of a successful fund-raising group called The Emily List (EMILY as in ''Early Money Is Like Yeast''), thinks that: ''Money is going to be a very difficult problem. One thing we've said to members is that we're going to recommend viable candidates. We have to see what case she is going to make that she can get the nomination.''

Schroeder is aware of the difficulties, especially financial, which is why she is ''just looking.'' And just being looked at. Or should I say listened to. But in one way Schroeder already compares favorably with rest of the guys. Her campaign pledge is easy to keep: ''I promise not to be boring.''