THE LATEST appeal to President Reagan and Congress to get the deficit under control is a remarkable event. The endorsement by five former secretaries of the Treasury has its own piquancy, given their collective contribution over the years to the difficulties they now urge those in power to remedy. Even more interesting perhaps is the public support now offered by several hundred people who run companies. It's fair to point out that many of the signers of the appeal are people who earlier enthusiastically applauded Mr. Reagan, his tax cuts and his supply-side strategy.

But a lot of minds have changed, and that circumstance gives greater force to their words now. They are not the familiar I-told-you-so. They are a warning to the White House from people who are, generally speaking, its friends and natural allies.

This "Bipartisan Appeal on the Budget Crisis" proposes a series of drastic remedies--a reduction of $60 billion in non-defense spending in the 1985 budget, a reduction of $25 billion in defense, and an increase of $60 billion in tax revenues. That would add up--with the resulting savings in interest costs --to about $175 billion, and produce, by the authors' calculations, a deficit that year of about $75 billion instead of the $250 billion that they fear is otherwise probable. To do anything less, they conclude, would be to invite financial conditions and investment performance in the 1980s that would be even worse than in the 1970s.

It is always possible to argue with the specific elements of a proposal like this one. Non-defense spending means, essentially, Social Security, the other pensions and medical care. To extract another $60 billion from those accounts over the next two years is probably not possible.

But the authors of this appeal have laid down the right set of basic principles. They emphasize fairness, and point out that the budget cuts of the past two years have borne disproportionately on the programs that help the very poor--food stamps, for example, and aid to families with dependent children. They give great weight to the encouragment of investment, for future economic growth is impossible without it. They urge a shift of taxes onto the money that people spend, and away from the money that they save and invest.

You will observe that they are talking about the budget year 1985--that is, the budget that will be before Congress throughout the presidential election campaign next year. The authors and organizers of this document--led by Peter G. Peterson, secretary of commerce a decade ago and now an investment banker--are offering the president good advice. Much more than the 1984 election depends on his response.