(FORTUNE Magazine) – The Keeping Up bookmaking department now makes it better than even money that the end of the Presidential Election Campaign Fund is nigh. As heavily hinted in the headline above, support for the matching-fund program will evanesce as one numbing thought swims into focus. The thought: A ''yes'' vote in that box on Form 1040 for 1991 will almost certainly be a vote to contribute taxpayer- generated campaign funds to David Duke, our country's best-known Nazi. To be sure, David is not the only embarrassment facing the matching-fund program this year. There is also Lenora Fulani, who in 1988 collected $922,106.34 for her campaign on behalf of the New Alliance Party and at year- end 1991 had already been awarded more money by the Federal Election Commission than any of the Democratic candidates except Tom Harkin. Lenora is a black activist lady who testifies proudly to Alliance Party support of and by professional troublemakers like Al Sharpton and Louis Farrakhan. But Duke is clearly the major embarrassment to the program. He is the only certified extremist who seems capable of turning his federal grants into a substantial number of votes. So he will be by far the toughest case for all the high-minded liberals who have long supported the grants as a wondrous reform, in the absence of which ''sewer money'' -- a somewhat tendentious term that keeps turning up in New York Times editorials endorsing the program -- would be dictating who was able to run in presidential elections. (There are no federal matching funds for politicians in state and local elections.) The Duke problem is surfacing at an awkward time for the program. As has been obvious for some time, the 1040 checkoff has relatively few fans. In the mid-Seventies, just after the program was put in place, around 28% of the taxpayers were voting yes. In the past few years the proportion has been around 20%. The 1992 presidential election is the first in which candidates have had to worry about actually receiving the amounts called for under the Federal Election Campaign Act. In general, the basic deal has been that candidates qualify for matching funds if they can raise at least $5,000, in individual contributions of $250 or less, in each of 20 states. If they meet that not terribly demanding standard, they are then entitled to get federal grants matching the first $250 of each contribution, up to a maximum grant of around $13.5 million per pol. But all this assumes that the Federal Election Commission has garnered enough taxpayer yes votes to pay the money due. If the FEC cupboard were empty -- as it might well be this spring and almost certainly would be in 1996 -- then the eligible candidates would have to settle for proportional shares of the money due them. A few politicians -- e.g., Republican Senator Mitch McConnell of Kentucky -- have already started demanding repeal of the law. It will be during the spring, we would judge, that calls for repeal will surface in volume. The basic lack of support for the program, as evidenced in the declining proportion of yes votes, will provide a suitable background for such calls. So will the manifest absurdity of any claims that the program has got the special ; interests on the run. Federal money for David Duke will provide a compelling last straw. The morning line is 3 to 5.