THIS FALL, about 100 of the nation's 435 congressional districts will have seriously contested House races. Typically, the Republican candidate will spend $400,000 and the Democrat half that. Is the deck stacked?

There are two distinct reasons for the Republicans' advantage. One is that they have done a better job of raising money -- the Republican groups had raised $161 million by the summer, Democratic groups about $25 million. The Republican National Committee has 1.7 million names on its direct-mail lists; the Democratic National Committee as recently as 1980 had 25,000. The deck isn't stacked against the Democrats here: there are millions of Americans who regularly support Democrats and who could afford the $15 to $100 contributions that direct-mail drives seek. Only now are Democrats seeking them out.

The other reason for the Republicans' advantage is that their party has proved more adept at raising money in open seats and for challengers to incumbents from the burgeoning business political action committees. Democratic House incumbents raise about as much from business PACs as their Republican counterparts; Democrats still chair House committees and subcommittees, and business PACs are eager to buy access. Business PAC contributions to Republicans, on the other hand, are directed, often with the guidance of party officials, to candidates in the seriously contested races. Labor PACs, by the way, contribute considerably less than business PACs, and the gap will probably widen.

No one can deny that money is very useful in campaigns; as a practical matter, you simply can't communicate with voters without it. But the candidate with the most money doesn't always win; if that weren't true, the Democrats would have lost every election from 1936 to 1960. The critical question is whether a candidate can raise enough money to communicate. The $200,000 or so the typical Democrat seems likely to spend in most seriously contested districts is probably enough; and if the typical Democrat fails to raise that much, it is at least partly his own and his party's fault, not the system's. The facts so far don't seem to us to indicate that the deck is irrevocably stacked against one party. But the subject needs a second look, after the campaign. Stacked Deck?