THERE IS one great omission in the respectable array of legislation that the White House and congressional leadership are moving toward passage: campaign finance.

Earlier in the Congress the Democratic leaders of both houses and the president suggested that this was a priority. But the president was mainly playing defense, and the Democrats have developed a strange way of demonstrating their enthusiasm. Neither Senate Majority Leader George Mitchell nor House Speaker Tom Foley has brought a campaign finance bill to the floor. Mr. Foley doesn't even have one to present yet. Sooner or later both will bring the controversial legislation up -- they're committed to -- but by then it is likely to be too late; it probably already is.

The problem of campaign finance is different in the two houses. In the Senate the issue is mainly that office costs too much: an average of $4 million a seat in the last election, and in some states two and three times that. The Senate dependence on outside money is compounded by the fact that that body has not abolished honoraria either. The House abolished these payments from interest groups last year.

In the House, the finance problem is partly the cost of office and even more where the money tends to come from. For too many members, powerful senior ones especially, too much of it comes from PACs, the giving arms of the interest groups whose legislative fortunes the members control. Some members are given so much they can't spend it all and have accumulated huge and, to any opponents, intimidating reserves. The S&L debacle (for which the executive branch also bears heavy responsibility) is a dramatic example of how the complaisant system can break down.

The first need is for spending limits. The Supreme Court has said that for First Amendment reasons these have to be voluntary, which means there have to be inducements through some form of public finance, whether in cash or in kind, to abide by them. There need to be limits as well on the aggregate that any member can accept in a cycle from PACs and on the resort to so-called soft money -- contributions outside the regulated channels. Any PAC and soft-money limits will end up being more deflective than airtight; there's a limit to how intrusive such regulations either can or should be.

The Senate Republicans don't like spending limits, which they say (we think wrongly) would hurt challengers and therefore their chances of regaining Senate control. In the House, meanwhile, well-kept Democratic incumbents (and Republican as well) don't want to give up their PAC money, and professionals in both parties -- but perhaps Democrats most -- worry about the loss of soft money. A deal is there to be struck, however, that could benefit the system while putting neither party at a disadvantage. The squalid alternative is a national legislature even more dependent than the current group on outside funds.