Only hours before he was shot, Ronald Reagan demonstrated the indispensability of his own presence and convictions to maintaining his administration as an instrument of revolutionary change in American government and American life.

At 8:30 a.m. Monday, President Reagan breakfasted with key officials of the administration. He exhorted them not to be sidetracked by political considerations in carryout the economic plan but to remember that this was one administration specifically mandated to make deep and permanent changes. "I got the impression that he felt that if we didn't agree with that," one policy-maker told us, "that we had no business being there."

The assassination attempt that afternoon left those who share Reagan's dream cold with fear at the futility of going on without Reagan. There is no alternative to Reagan himself to ensure that his goals are not diluted into the fuzzy pragmatism of the previous 16 years of postwar Republican administration.

The administration of Dwight D. Eisenhower, with whom Reagan often is compared, functioned efficiently without apparent disruption during three prolonged absences of the ailing president. While Reagan indeed resembles Eisenhower in wholesale delegation of duties, he alone has set the ideological tone of his administration in a way the old general never did.

Accordingly, the announcement Reagan would be able to function as president the very day after his shooting and might leave the hospital within two weeks was of vital importance. Nobody could guess what would happen to this administration's motive force during protracted convalescence for its chief. Far from being the irrelevancy of his caricatures, Reagan is the vital spark that moves his administration.

Vice President George Bush has gained Reagan's confidence, as witness his triumph over Secretary of State Alexander Haig in their power struggle. hBut even if Bush, with vastly more governmental experience than Reagan, fully agreed with Reagan's revolutionary goals, he could not match Reagan's ideological commitment. The White House senior staff, including the president's longtime servitors, seem closer to Bush than Reagan in lack of ideological intensity.

Nowhere is this clearer than in the Reagan tax cut. When Chairman Dan Rostenkowski (D-Ill.) of the House Ways and Means Committee was quoted (incorrectly, Rostenkowski later said) as saying the three-year tax rate reduction bill was dead, the president was furious. That fury was not discernible in either his senior aides or his vice president, who had campaigned hard against Kemp-Roth while he was a candidate.

On the day of the shooting, plans were under way to invigorate the tax program with a Reagan speech to the nation. The president's incapacitation obviously delays that important effort, but probably not for long. A more serious injury would have removed, with critical effect, the administration's single most effective advocate for tax reduction.

Those anguished hours on Monday (inexpressibly worsened by the grave wound suffered by Jim Brady), when nobody knew the extent of the president's injuries, suddenly made his ideological supporters aware of the absence of true votaries in his administration. The closest is budget director David Stockman. But at age 34, Stockman lacks the president's constancy. In his quest to balance the budget, Stockman has flirted with tax gimmicks and seemed cool toward defense increases; Reagan has had to set his young disciple straight.

Less than a week before the gunman struck, Reagan demonstrated the uniqueness of his interest in ideas and philosophy. The council of non-government economic elders assembled here March 4 to consider the administration's drift toward protectionism on Japanese auto imports. To the man, the economic elders were against it.

Prof. Milton Friedman was scathing, pointing out that the impact of quotas was equally disastrous for free-market economics whether they were "voluntarily" accepted by the Japanese government or imposed by act of Congress. The president nodded his head, indicating agreement with Friedman that the administration had gone wrong. He had been expected to make that clear this week.

No other recent president would have been so moved by a word-famous ideologue on the trade issue, where governments historically follow considerations of Realpolitik, both domestic and foreign, not ideology. Nor is it likely that George Bush would. But Reagan himself is no less an ideologue than Milton Friedman.

That is what makes the president personally irreplaceable if his administration truly is to change the nation. Confined to George Washington University Hospital, his administration's vital force is depleted. If he had been killed or incapacitated, its radical quality would have ended.