Debate on the 1990 farm bill is drawing to a close with surprising mildness. Agreement is more frequent than argument and the rancor quotient is low.

The reason, say many congressmen, is that the worst is yet to come. Congress would like to spend up to $54 billion in farm benefits over the next five years, but there is no chance of that.

The budget crisis has overshadowed the farm bill since its conception months ago. It has muted congressional rhetoric, cooled tempers and made hash of ideology.

The message is clear: "Let's be honest to our farmers," Senate Minority Leader Robert J. Dole (R-Kan.) told his colleagues. "We need a budget agreement and agriculture will have to do its part."

The Senate yesterday finished the sixth day of deliberations on its version of the bill, while the House prepared for its third day of debate today.

Both houses of Congress are working with budget estimates of $53 billion-$54 billion in crop subsidies and benefits spread over the five-year life of the bill, but the funds are certain to be chopped when the current budget summit finishes work.

By how much? Democratic Party sources peg the cuts at $3 billion to $10 billion over five years; Republicans estimate $4.5 billion-$5 billion; Dole said $6 billion-$18 billion; and the Bush administration said cuts could reach $20 billion.

Whatever the final figure, there is general agreement that most of the money will come from farm subsidy programs. This will involve what one Democratic Party source called "major structural changes" in how payments are made.

Cutting the subsidies that are the heart of any farm bill will probably take place after the Senate and House pass their respective bills and meet in conference to hammer out the final version. It will be a far more painful task than anything so far.

In fact, the knowledge that the worst is to come has probably muted criticism during floor debate.

"The budget has caused us to proceed with more discipline," said Rep. E. (Kika) de la Garza (D-Tex.), chairman of the House Agriculture Committee. "It's had an impact on the ideology."

In the House, de la Garza has enjoyed a bipartisan, clubby atmosphere that helped resolve knotty questions before they came up for consideration.

In the Senate, however, the careful coalition fashioned by Agriculture Committee Chairman Patrick J. Leahy (D-Vt.) and ranking minority member Richard G. Lugar (R-Ind.) showed signs of strain.

Leahy, a moderate from a largely non-farm state, led a restive group of Democrats, among them several midwesterners anxious to amplify benefits to small family farmers.

Lugar, a patient man with a homespun demeanor, represented the Bush administration's free-trade, hold-the-line-on-spending Republican mainstream. He was seconded by Dole, who appeared infrequently but memorably as the bearer of bad tidings: big spending would mean tragedy once the budget summit completed its work.

At one point Dole announced he had a "substitute" farm bill, but Republican sources said he would probably wait until the conference committee met before introducing it.

The Leahy-Lugar compromise met its sternest test Tuesday night in debate on an amendment introduced by Sen. Thomas A. Daschle (D-S.D.) to change the subsidy structure.

In an eloquent exchange, Daschle, a youngish, earnest exponent of higher benefits, made an impassioned plea on behalf of small farmers.

"We've been told time and again that only if the budget would allow, we could get a program that could ensure small farmers long-term viability," Daschle said. "As we divide up our shrinking pie, don't we want to ensure that those who need it most get it?"

Daschle said he had tried unsuccessfully to raise subsidies earlier in the debate, and he understood that "we're running an uphill battle. But it's disconcerting to drive across a state like mine and know that next year a lot of small farms won't be there any more."

Lugar responded that he thought the Agriculture Committee had agreed that subsidies would not be raised. Anything else was "a non-starter," he said.

"We're talking about a very serious effort here to bring in a bill that satisfies our budget concerns," Lugar said. Raising subsidies "is not going to happen. It's going to crash in a heap."

The Daschle amendment, he concluded, was a "deal breaker." He said he had told Leahy this and Leahy had agreed to oppose the amendment. Leahy did, and the amendment went down 72 to 24.

"The farm bill is a Rubik's Cube, because of the budget," Leahy said, referring to a puzzle of interlocking pieces. "I believe the cube would begin to fall apart if we changed a face."