THE GREATEST undone deed of the last Congress was the one that would have struck closest to home: campaign finance reform.That is not an accident; each party fears some aspect of a cleaner system. The Republicans are the better-known opponents; they object to the spending limits without which there can be no reform. But the House Democrats, the most entrenched group in national politics, constitute an obstacle as well. They passed a bill in August, but it was partly a shell and they didn't mean it. The latest figures on campaign finance suggest why.

Of 405 House members seeking reelection, 78 had no major party general election opponents as of Sept. 30, 218 had opponents who had raised less than $25,000 and 86 had opponents who, while they had raised more than that token amount, still had available fewer than half the resources of the members they were trying to unseat. That's a total of 382 incumbents or 94.3 percent of those running who in financial if not electoral terms were without serious opposition, Common Cause, which compiled the figures, observed. The incumbents entered the election season with $177.5 million in campaign funds, their challengers with $14.8 million. Of the incumbents' money, $58.2 million was left over from previous elections -- increasingly they are given more than they can spend -- and another $57.2 million came from PACs; PACs gave challengers only $1.9 million.

The members can be forgiven if they think this is the perfect system; from the standpoint of their perpetuation in office, it is. Republican incumbents benefit as well as Democrats, but they have the incentive of having been so many unhappy years in the minority that some want to blow the system away. The bill that the Democrats pushed through the House on a party-line vote had some good and even imaginative features, among them a federal tax credit for small in-state contributions to candidates who agreed to abide by the spending limits it prescribed. But the huge carry-overs from past years and the steady supply of PAC money are what make House races non-competitive, and the bill did not seriously address either of those.

In the Senate the problem is in some ways simpler, not so much where the money comes from as how much is required. In the last election the average winner spent more than $4 million some (in larger states or with strong opponents) spent two or three times that. To accumulate just the average a senator has to raise $12,000 a week every week of a six-year term; they live with their hands outstretched. The limits the Democrats passed would ease the chase, and it was possible to see in the Senate the outlines of a bipartisan agreement.

The Senate should pass its bill again early next year. It will then be up to the House Democratic leadership both to negotiate more seriously with the Republicans than they did this year and to produce a stronger bill in time for a conference to produce the legislation well within reach that both parties can live with and that will clean up the system. No task is more important; the present system is to the long-run advantage of neither party. In both House and Senate it produces -- and will again produce tomorrow -- a Congress dangerously close to being bought.