A constitutional amendment that would make it harder to unbalance federal budgets was attacked yesterday as a cowardly way of solving deficit problems, but critics acknowledged it probably will pass this year.

Senate opponents began their uphill fight to derail the measure by offering major changes that would transform it from a constitutional amendment into a simple statute governing Congress.

Leading that charge, Sen. Charles McC. Mathias (R-Md.) called the amendment a "fig leaf" to cover up Congress' lack of restraint in approving a $100 billion deficit for next year, and predicted it wouldn't fool anybody.

"The American people are bound to wonder how the same Congress that brought back a budget $100 billion out of whack can sit here with our hands piously folded in front of us and vote for a balanced budget," he said.

In the House, Speaker Thomas P. O'Neill Jr. (D-Mass.) called it a "cowardly way out" but acknowledged it probably would pass the House this year. Senate passage is considered nearly assured, but the Democratic-controlled House has been a stronger bastion of opposition.

O'Neill also said he would not seek to block an effort to discharge the amendment from the House Judiciary Committee, which has refused to bring it out for a vote. The chairman, Peter W. Rodino Jr. (D-N.J.) is strongly opposed to it.

The amendment has strong appeal in an election year because of its promise of future balanced budgets when Congress just approved a 1983 budget resolution calling for a deficit of more than $100 billion.

But some had considered it likely that the House leadership and Rodino would be able to keep the amendment bottled up and off the floor until Congress adjourns for the fall elections.

O'Neill's comments yesterday indicated that is not likely to happen. "I am not going to try to keep it off the floor," he told reporters.

In the second day of Senate debate, opponents got their first crack at the amendment, which would bar Congress from passing a budget with outlays exceeding revenues, except by a special vote of three-fifths of both houses.

The major vehicle used by opponents is the amendment by Mathias and Sen. Max Baucus (D-Mont.) which transforms it into a simple statute but which also would require a three-fifths vote to approve a deficit.

Mathias said the constitutional amendment process, requiring ratification by 38 states, would take too long to rescue Congress from its current deficit problems and claimed a statute would have the same binding effect.

But the chief sponsor, Sen. Orrin G. Hatch (R-Utah) observed that Congress already has a statute banning deficits--one that is routinely ignored. The "institutional bias" in Congress to spend more than the revenues available can be overcome only by the "outside force" of a constitutional amendment, he said.

Sen. Pete V. Domenici (R-N.M.), chairman of the Senate Budget Committee and a key figure in budget debates this year, agreed the amendment might increase the pressures to pass balanced budgets. But he said he will introduce five changes to "improve" Hatch's version. Hatch said Monday he will oppose all changes.

Domenici warned that the ban on deficits could be circumvented by congressional devices, such as resorting to more loan guarantees that do not add to deficits or simply transferring much of the spending to a "capital budget" which would not be included in calculating operating deficits.