BEFORE CONGRESS now is a proposal that is the legislative equivalent of the Veg- o-matic -- you know, the gizmos advertised on late-night TV that chop onions and grate carrots and core apples, almost without your having to lift a finger. The legislative equivalent is being peddled now as the easy cure for the deficit, the national debt and ever-burgeoning spending. It is the line- item veto.

Imagine, proponents say: it could enable the president to cut wasteful spending with a stroke of the pen. And if you're not sure it'll work, they add, just give it a 365-day trial. But at this point you may have a few questions.

How is a president going to cut spending much through line-item vetoes when no recent president has proposed anything close to a balanced budget? President Reagan might like you to think Congress has been spending wildly. But Congress has authorized and appropriated only marginal increases over his own budget. If a president as economy-minded as Mr. Reagan dares not propose further cuts in his budget, why would he impose them through a line- item veto?

Another question: How would you stop Congress from doing what it does now -- putting the spending the president doesn't like in the same line with items he dare not veto? The bill introduced by Sen. Mack Mattingly (R-Ga.) would try to stop this congressional practice by commanding Senate and House clerks to enroll each individual "item" as a separate bill. But it won't take a very skilled parliamentarian to get around this requirement. Certainly Congress has a powerful institutional reason to try. Otherwise, the line-item veto would shift vast power from Congress to the White House and the Office of Management and Budget.

In the long run, there is no simple, fail-safe, automatic device to force a president and Congress to do what this president and Congress and their predecessors have refused to do: balance the budget. The line- item veto, like the constitutional amendment purporting to require a balanced budget, is a political gimmick, certain in the short run to be mischievous and in the long run to be ineffective. The president and Congress already have the constitutional power to balance the budget. What they need is not a line-item Veg-o-matic, but political will.