Skip the penumbras and forget the emanations. The Supreme Court got back to constitutional fundamentals on April 27 when it heard argument in the case of the line-item veto. Is this ugly duckling a constitutional swan?

No way. The act is as unconstitutional as a third term for William Jefferson Clinton. That's my opinion anyhow, and if the high court gets to the merits of the issue, my guess is that the Supremes will say as much. They will affirm the lower court opinion of Judge Thomas F. Hogan. He ruled in February that the line-item veto violates the venerable principle of separation of powers.It is quite possible that the court will never get to the merits at all. Two cases have been consolidated. The City of New York brought one of them. The Snake River Potato Growers of Idaho brought the other.

Both the Big Apple and the Big Potato say they MAY be hurt, or COULD be hurt, or are LIKELY to be hurt by the president's action. They concede that they have not been hurt so far. Still, you never know, and why take a chance?

This is not the kind of posture that finds favor at the high court. New York City's challenge to the line-item veto is tangled in a briar patch of regulations governing federal payments to the state of New York for Medicaid. President Clinton's veto evidently would cost the city a bundle - or it MIGHT cost the city a bundle - and the city is aggrieved by the prospect.

In Idaho, the potato growers' cooperative would like to acquire processing plants. Section 968 of a tax bill would enable the co-op to compete evenly with private companies in bidding on available properties. With the veto the farmers have lost that level playing field they were promised, and they want it back. It has to do with the taxation of capital gains.

This is the key constitutional provision: "Every bill which shall have passed the House of Representatives and the Senate, shall, before it become a law, be presented to the president of the United States; if he approve he shall sign it, but if not he shall return it, with his objections to that House in which it shall have originated . . . "

The argument over a line-item veto goes to one word: "it." The Constitution does not permit a president either to sign or to veto merely part of a bill. His power is to sign or to veto "it," which is to say, the WHOLE bill.

Two basic principles support the Constitution. One is the principle of federalism, which is not at issue here. The other is the principle of separation of powers. It is this principle that the line-item veto grossly violates, for the line-item veto act transforms the president into a super-legislator.

The line-item veto also tramples upon the basic political principle of compromise.

The line-item veto act applies only to a small fraction of all federal spending. Properly invoked, the law might save us a billion here and a billion there. As Sen. Everett Dirksen observed, pretty soon this can add up to real money, but we have no business economizing at the cost of constitutional principle.