THE BUDGET negotiations to be held today are described as crucial. That's a slight exaggeration. Even if the discussions move toward agreement, the inherent difficulty of rewriting the budget means that there are still many risks and hazards ahead. Even if the discussions collapse, the White House and Congress will have to keep returning to the enormous deficits implied by Mr. Reagan's February budget, and the damage that they are threatening to inflict on the whole economy. Perhaps that's the thought for the negotiators to keep in mind if things begin to go badly--that the deficits will have to be brought down sooner or later, one way or another, and delays can only increase the costs.

Interest rates are not going to come down as long as the president's budget shows large and steadily increasing deficits marching out into the future as far as the eye can see. It's as simple as that. To bring next year's deficit below this year's will require a combination of substantial changes in the Reagan budget, all of them painful. It will require higher taxes, lower defense spending and some temporary shaving of the Social Security cost-of-living increases. Mr. Reagan is not eager to do any of those things. Hardly anybody wants to do all of them. But it will take all of them together, and more, simply to keep next year's deficit below this year's.

Mr. Reagan's conduct has been unusual throughout this whole process of budget negotiation. He has denied stoutly that they were actually negotiations, or that the administration was taking any active part. He has consistently talked as though he saw no need to change course at all. If those quarrelsome people in Congress had anything to suggest, he'd be glad to look at it--but that is as much as he has conceded so far. Now he is going to have to do much more. Unless he is prepared to commit himself fully and publicly to very large revisions in his February budget, there will be none. Those revisions will be too difficult and, in political terms, too dangerous, for Congress to undertake without his full support and cooperation.

Most of the public and visible leadership toward budget revision has been coming from the senior Republicans in the Senate. Their interest is obvious. But what about the Democrats who are the majority in the House? Why would they deliberately run large risks, and cast unpleasant votes, to rescue a Republican president from his own miscalculations? The only answer is that the national interest has become compelling. It's not only Mr. Reagan who would be rescued, but, in addition, the businesses and jobs throughout the country that may otherwise vanish if misconceived budget policy prolongs the recession.

It's a strange situation. Congress can't put the budget back on track without the president, and the Republicans can't do it without the Democrats. At best, there's no more than an outside chance that it can be brought off. Only one thing makes this endeavor worth considering at all, and that is the thought of the effects of another five or six months of interest rates at their present levels.