The Federal Election Commission (FEC) has issued another of its periodic warnings about the lack of public funds available for presidential campaigns. This time, however, the FEC is warning that there could be a shortfall for both the 1992 and 1996 elections.

FEC auditors said the Presidential Election Campaign Fund's deficit could be $6 million by the end of the 1992 election year. The overall projected cost for the 1992 primaries, conventions and general election is $187 million.

For the 1996 election, the FEC estimates that, without some remedy, the fund's shortfall could be more than $150 million.

At the heart of the shortage is a decline in participation in the program that lets taxpayers designate on federal income tax returns that $1 of their taxes go directly to the campaign fund. Participation has dropped steadily from a high of 28.7 percent in 1980 to 20.1 percent in 1988, when taxpayer checkoffs supplied $178.2 million for election activities.

The FEC is studying several plans to shore up the fund, including indexing the $1 checkoff for inflation, eliminating convention funding, which was more than $9 million for each party in 1988, or funding the campaigns through appropriations rather than direct taxpayer participation.

But FEC Chairman Lee Ann Elliott, warning that the projected deficit could have a serious impact on presidential campaigns, said, "A legislative solution is the only way to ultimately solve the inherent troubles of the presidential fund."

In the meantime, the commission is interviewing focus groups in Portland, Ore., Fort Lee, N.J., and Chattanooga, Tenn., to study why taxpayers do not participate at higher levels. The FEC also plans a public education campaign on the presidential fund, reminding taxpayers that the amount of their tax or refund is not affected by marking their tax return to designate a contribution.