PRESIDENT CARTER used familiar and fashionable language the other day in endorsing S. 926, the pending bill for partial public financing of Senate campaigns. He said the bill would "help restore the public's confidence and trust in officials" by removing "the appearance of obligation to special interests." Now, that sounds soothing - but this is no time for the Senate to relax. There is more than cosmetics involved here, and the effects of this legislation in its present form might not be as restorative as the advertising suggests.

Consider the matter of "special interests," which has become shorthand for corruption. Any two people could argue all day about which interest groups - bankers or doctors or unions or whatever - are "special" in the pejorative sense, and what their political role should be. Regardless of where one comes out, two points seem clear to us.

First, the most corrosive kinds of interest-group money, the huge, often covert donations such as those revealed in recent years, either have been illegal for decades or were curbed by the disclosure rules and contributions limits enacted in 1974.

Second, the role of political-action committees and big donors would not necessarily be reduced a whit by S. 926. Public matching of small private gifts would give Senate nominees less need to court big contributors. But the bill does not cover primaries, where some elections are settled and early, big donations can have the greatest effect. Moreover, as the Supreme Court emphasized last year, the First Amendment gives individuals and groups the liberty to spend as much as they want on independent, parallel campaigns for candidates. Last fall, while Mr. Carter's election drive was being publicly financed, labor unions were spending, by one estimate, over $11 million independently on his behalf. So when Mr. Carter says that public financing for presidential campaigns "worked very well last year [pause . . . laughter]" without any of the candidates "being obligated to anyone," we get the joke - but not the argument.

The advocates of public financing also claim that it would open up the system and, in Mr. Carter's words, "help enable deserving candidates to run for office even if they are not rich." This also sounds good - but also raises large questions about the nature of political competition and the proper role of government.

It's true that competition is inhibited by the high and rising cost of getting political messages into the marketplace at all. If a candidate can't afford advertising, voters have no way to gauge whether he is "deserving" of support. Especially in primaries, one can justify more public-service broadcasts, publicly financed mailings and perhaps modest matching grants. But public-financing's advocates have much more in mind. They want to assure challengers not just basic access but equal funds. S. 926 would promote parity by setting spending limits as a condition of public aid. And if one candidate foregoes that aid and exercises his right to spend more in private money, his opponent would get extra subsidies.

What's wrong with this? For one thing, it reflects a simplistic view of the role of money in campaigns. More dollars don't always mean more votes. in the past five years, 17 men have come to the Senate the hard way, by beating incumbents; nine of those 17 won even though they were outspent. Beyond that, S. 926 would legislate a value judgment: Big spending is bad. But if a candidate with wealth or access to large sums does spend millions, as long as his financing is lawful and fully disclosed, who should decide whether that's fair or wholesome? Since the record suggests that lavish spending is not necessarily decisive and can itself become a campaign issue, why not let the voters make these judgments on a case-by-case basis?

That leads to the rock-bottom question: To what extent should government regulate political activity? Public financing of presidential campaigns, of course, also poses that question and in time may help to answer it. in our view, that experiment worked well enough last year to be continued in 1980. It was, however, limited to contests for one nationwide office. And it did affect the structure of politics, especially the nature of fund-raising and the role of political parties, in ways that are not yet entirely clear. The problem of dealing fairly with third-party and independent candidates remains unsolved and may be unsolvable.

Finally, the whole regulatory effort launched in 1974 has produced not just administrative burdens and snarls but also a worrisome degree of official involvement in the details of political practices. That, more than anything else, suggests that Congress should not rush into public financing for Senate and House campaigns across the land. A law meant to return politics to the people would defeat its own purpose if it turned out, instead, to be dictatorial and stultifying.