Remember when Congress came up with an innovative way to help reduce the undue influence of a few so-called fat cats on politics by letting the taxpayers voluntarily foot part of the bill for presidential election campaigns?

We're referring to the check-off on federal income tax returns, where a taxpayer can assign $1 of his or her taxes to a presidential campaign fund without adding to his or her tax bill.Well, 17 years have passed since that law was put on the books. Now the time has come to take that law back to the shop for some long-overdue repairs. But not the ill-advised tinkering the U.S. Treasury Department has in mind.

Treasury is acting because contributions to the presidential campaign fund have been off lately - only $32.5 million last year, down from the peak of $41 million in 1981.

As a result of this trend, the fund is expected to have accumulated just $127 million by next Dec. 31. That amount, the New York Times reports, is not expected to be enough for full allocation to presidential candidates running in the 1992 primary elections - and it's the early primaries when the funds are needed most.

Unhappily, the Treasury Department is only compounding the problem by proposing that next Dec. 31 be the cutoff date for accumulating the tax check-off money to be distributed in the 1992 campaign. What folly! By also counting the funds expected to come in by the spring of 1992 from 1991 tax returns, Treasury would have another $30 million to allocate.

An even better reform would be to adjust the check-off provision for inflation, letting taxpayers devote not just $1 but $2 or $3 for the presidential campaign.

The best reform, though, has nothing directly to do with the check-off provision. We're talking about the need to stop letting presidential campaigns drag out over months and even years. Instead, they could and should be completed in only a few weeks. Because they would be less costly, shorter campaigns would offer less opportunity for a few wealthy political contributors to exercise excessive influence. Shorter campaigns would also be less emotionally draining on candidates and voters alike.

Britain and Canada regularly complete national political campaigns in only five or six weeks. Why can't the United States do so, too?