THE IDEA THAT MONEY decides elections is being tossed around again by advocates of public financing of congressional campaigns. Last week Mark Green of Congress Watch, a Nader group, bemoaned the fact that in 28 of 33 Senate contests this fall, the candidate who spent more money won. To Mr. Green, this has "very serious" implications for democracy.

Setting the implications aside for now, let's look more closely at the facts. Is spending always - or even usually - the decisive element in a campaign? You can't prove that from the Senate results only two years ago, when the bigger spender won in 16 contests and lost in 15. Even this year, the exceptions are notable. The five "bigger spenders" who lost anyhow included four of the seven imcumbents who got beaten - Democrats Thomas McIntyre (N.H.) and Dick Clard (Iowa) and Republicans Edward Brooke (Mass.) and Robert Griffin (Mich.) - plus Robert Short of MInnesota, whose lavish spending of his own funds funds no doubt contributed to his defeat.

Of course campaigns have to have money. But how much is adequate - and how much more makes how much difference? Those are intriguing questions for which general answers may not exist. According to reports so far, the Senate winner in Nebraska, Democrat James Exon, spent about $170,000 against his opponent's $159,000. That $11,000 gap hardly explains why Gov. Exon got 68 percent of the vote. Or take the most egregious "big spending," Sen. Jesse Helms' $6.7-million campaign. Against an opponent with $217,000, Mr. Helms got 54 percent of the vote - about what he won six years ago. Would a million more or less have changed that result? Which way?

The subject is befogged by speculations. One fact, however, is clear: campaigning has become horrendously expensive. No matter what one's opponent spends, a non-incumbent without personal wealth or generous backers has a hard time getting his message around. Here's where the reformers could do some helpful work by sorting out the kinds of campaign expenses, including fees paid to consultants and direct-mail operations, and suggesting how costs could be cut. That's one way to promote competition without imposing controls. Besides, it's often said that half of all campaign money is wasted - and even big spenders would love to know which half that is.