President Carter may end up facing a politician's nightmare at the start of the 1980 presidential race: Keep his old campaign promise to balance the federal budget in fiscal 1981, or risk violating the law.

Vitually unnoticed, a House-Senate conference committee mandated a blanced a balanced budget in an otherwise-unrelated bill approving U.S. participation in International Monetary Fund effort to aid financially troubled nations.

Now, Carter must sign the bill - with the balanced-budget rider in it - or risk jeopardizing the IMF legislation and wrecking the plan to aid deficit-ridden countries. The bill squeaked through narrowly as it is.

Carter faces two uncertainties. He doesn't really know whether the rider means what it says in terms of requiring a balanced budget. And he doesn't know how whether he can balance the budget in 1981.

The amendment was slipped through by a longtime balanced-budget advocate, en. Harry F. Byrd Jr. (Ind.-Va.), who called it vital to restoring national economic health and Vitality.

The provision would seem straight-forward enough. It reads: "Beginning with fiscal 1981, the total outlays of the federal government shall not exceed its receipts."

But that language could be diluted by the statements of the conferees and assertions in the conference report, Sen. Jacob K. Javits (R-N.Y.) calls it "an expression of an aspiration." And the report warns it "may be superseded."

As a result, neither the lawmakers nor the administration is quite sure what, if anything, the provision requires. "We're not sure what it means," a top White House analyst confesses. "But it looks a little ominous."

It was so difficult to pass the IMF bill that Carter can't afford to risk a veto. Insiders say the president "now intends to sign the bill" within 10 days.

Officials say the president will try to get out of the box by issuing a statement when he signs the bill "clarifying what he believes the provision means." Such a statement may not have the force of law.