AS CONGRESS HAS moved toward balancing the budget, the drive to bar deficits or curb spending by constitutional amendment has gotten somewhat obscured. it is still puttering along; its sponsors claim that 30 state legislatures have now called for a constitutional convention if Congress fails to submit an amendment to the states. Yet like many other "grass-roots" movements that burst into general view, this one has had some impact as a protest, but has failed to stand up well under sharp scrutiny.

From the start, the amendment-or-convention drive was grounded in frustration with a free-spending Congress that seemed oblivious to popular concern about inflation or huge deficits. But by the time the campaign became the center of much attention last winter, Proposition 13 and other messages had already turned Congress toward restraint. The convention threat probably did discourage backsliding this spring. It also helped propel the Senate and House into explicit commitments to a balanced budget in fiscal 1981. On the other hand, it also provoked some unhelpful congressional hostility toward the states and put some sound federal-aid programs - especially revenue-sharing - into at least temporary jeopardy.

Yet Congress' endorsement of a lower deficit and a balanced 1981 budget have also been spurred by two factors that the sponsors of an anti-deficit amendment do not appreciate. One is inflation, which has enabled Congress to increase its revenue forecasts and thus cut the deficit without reducing overall spending much below President Carter's proposed level. The second is the decision to forgo a tax cut next year and probably in 1981 as well.

Thus, some who favor permanent curbs on spending are still pressing for a constitutional amendment as a guard against the day when the currents of politics and economics may no longer run toward restraint. Yet this year's experience ought to show the difficulty of drafting - much less enforcing - such arbitrary curbs. This year, for instance, Congress had good reason for the adjustments in its estimates that have produced a politically happy result. Faced with a constitutional demand for balance, however, Congress might well act much less responsibly and come up with a paper balance that obscured all sorts of inflated gains and hidden expenditures. In short, an amendment could easily work against the kind of responsible budgeting that Congress is now lning to do.