IT IS FAST becoming conventional wisdom, but is nonetheless true, that heavy political contributions by PACs to well-placed congressional incumbents are giving politics an even worse name than it already had. In the last two-year election period, PACs contributed more than $100 million to congressional incumbents, and you can bet that not all that money was given out of abstract admiration for the congressman's record or a disinterested desire for good government. PAC organizers tell their constituents that their contributions are getting them something, and many people around town and around the nation are getting the sinking feeling that they're right.

What should be done? Various answers are circulating around Capitol Hill, all of them in tentative form. One is a proposal by Sens. Charles Mathias (R-Md.) and Paul Simon (D-Ill.) for optional public financing of Senate general elections. Their bill would allow major party nominees for the Senate to opt for between $500,000 and $5.7 million of public financing, depending on the size of the state, for their general election campaign. Candidates who accept could accept no individual or PAC contributions and could spend no more than $20,000 of their personal money.

The Mathias-Simon bill falls far short of a complete public financing system and would supplement the existing public financing schemes we have now, in the presidential races and in elections for governor in Michigan and New Jersey. Those limited versions of public financing have worked out fairly well, but it should be remembered that each is tailored to the peculiar circumstances of a particular kind of contest, each has had its share of glitches, and each in important respects seems now to be in need of revising.

All of which raises the question of whether any single public financing formula, even one as simple as that in the Mathias-Simon bill, can work acceptably -- can stimulate competition and operate fairly -- in 50 different states and in Senate elections with very different kinds of competition. The Mathias-Simon bill does little for states -- and in a given year there still are some -- where the real competition for a Senate seat comes in the primary. The amounts mandated may be so low as to put candidates who accept them at a serious disadvantage to those who don't. The source of financing, the $1 federal income tax checkoff also used for public financing in presidential races, may not be sufficient.

Such are the kinds of questions that should be raised and objections that should be made to any proposed campaign finance reform. It is a devilishly difficult business, and the unintended consequences of reform can prove to be far more important than the intended benefits if a new law is not scrutinized. The Mathias-Simon bill should be considered in this context.