THE MARTIAN parked his saucer in a nearby orbit the other day and stopped by for a cup of coffee. He had big news. A publisher has given him an advance to write a book about the American election next year. We wished him lots of luck.

Being from Mars and all, he said, he's a little weak on background and he wanted to get up to speed on the tax issue. We offered to share our resources.

"From the papers" the Martian said, "I see that the Republicans are in favor of a big, fast tax cut, to let the country spend its way back to prosperity." That seems to be the case, we agreed.

"Meanwhile," he said, "the Democrats have returned to their traditional position as the party of balanced budgets, fiscal discipline and tight money." "Traditional?" we inquired. It turned out that he had been reading the 1932 campaign speeches of Franklin D. Roosevelt. He had a point, we said. Perhaps the party is at last finding its way home.

But he noted that Sen. Lloyd Bentsen (D-Tex.), the chairman of the Joint Economic Committee, had joined the committee's ranking Republican, Rep. Clarence Brown (R-Ohio), in calling for a $20-billion tax cut right away. The Martian wanted to know what a good Democrat like Sen. Bentsen was doing in that kind of company. We explained that in Congress, the Southern Democrats often ally themselves with the Midwestern Republicans on economic questions.

"And who's this Mr. Greenspan, who is also calling for a $20-billion tax cut no later than January?" the Martian asked. We said that Alan Greenspan is a noted Republican economist who frequently speaks for the business community. "Isn't that the same Mr. Greenspan who was chairman of the Council of Economic Advisers in the Ford administration, when the budget deficit hit $66 billion? True, we said, it was the very same Mr. Greenspan, and it was the largest peacetime deficit in American history. The country was just coming out of a very severe recession in 1976, and any person of social conscience had to agree that a walloping deficit was necessary to get incomes up and unemployment down.

"But isn't your country heading back into another recession now?" the Martian inquired. "What's President Carter doing about it?" We said that he has adamantly cut off all discussion of tax cuts within the administration, and the Federal Reserve Board has squeezed interest rates up to record levels. The Democrats are the party of sound currency, we pointed out. The Republicans deliberately devalued the dollar, but the Democrats are pledged to defend it. The Martian wanted to know what Mr. Carter will do if the unemployment rate starts rising during the presidential primaries. We confessed that we honestly don't know. In fact, the more we think about the relationship between fiscal policy and the parties, the more confused we get.

The Martian smiled comfortably. On the contrary, he said, he found the subject perfectly straightforward. "To follow American politics," he said, "it sometimes helps to be a Martian."