THE BUDGET compromise among the Republicans was crucial. Without it, the whole federal budget process would have slid toward a dangerous kind of paralysis. The Senate leadership has managed to press President Reagan into an agreement. That's the first real step toward enactment of a budget, under the peculiar procedure on which Mr. Reagan currently insists.

The budget this year is following precisely the same route as last year. Once again Mr. Reagan has refused to send a serious budget to Congress. Once again he has left the initiative to a small group of courageous Republican senators -- most prominently Pete Domenici, the chairman of the Budget Committee, and Robert Dole, the majority leader. Once again there have been ardu- ous negotiations eventually arriving at the triumphant announcement of a compromise between this Republican president and his own party. That's what happened in the famous Rose Garden agreement a year ago, and it happened again last week.

The most significant improvement this time is Mr. Reagan's agreement to an increase in defense spending only half as large as his original request two months ago. But this compromise also contains serious defects. A one-year freeze in Social Security benefits would have been tolerable. But reducing them by 2 percent a year for each of the next three years is grossly unfair and an ominous precedent. Holding down the cost-of-living adjustments year after year means that retired people get poorer as they get older.

This Republican compromise also calls for a number of spending cuts that seem unlikely to get through the Senate with its Republican majority, let alone the House. You are entitled to doubt that, with the present level of distress among farmers, the Senate is going to choose this year for large reductions in the farm price-support programs. On taxes, the senators have given in entirely to the president. This year there will apparently be not even a token tax increase.

Since the budget this year is following a path closely paralleling last year's, it's worth noting how last year's story is turning out. Congress not only enacted the Rose Garden agenda but more, for a total of about $20 billion in deficit-closers for the fiscal year 1985. But meanwhile events have revealed a series of miscalculations and nasty surprises that have pushed revenue and spending $47 billion farther apart. The result is that, despite that $20 billion worth of good work by Congress, the administration now expects the 1985 deficit to be $27 billion larger than it estimated a year ago.

The moral of the story is that the Republican senators are pushing in the right direction, and they are owed great credit for that. But they will have to push much harder, with much more active help from the president, if they are to make any enduring difference in a deficit that is now a menace to the American economy.