THE PERSONAL PRESIDENT: Power Invested, Promise Unfulfilled. By Theodore J. Lowi. Cornell University Press. 221 pp. $19.95.

NOT THE LEAST amazing of the acts of legerdemain that Ronald Reagan has been able to perform in the White House is to pretend successfully that he is a new kind of president, sharply contrasted from all those big- government liberal presidents that went before him.

Now at last from Ithaca comes Professor Lowi to demolish that myth among many others. Reagan entered office dedicated to a smaller government and a smaller presidency, says Lowi, but what had happened within three years?

"In terms of the deficit and the size of the national debt, he had given us the largest government we ever had. And, what is more interesting, instead of a smaller presidency, he gave us one of the most prominent we ever had."

However laid back or "hands-off" he is, Lowi comments sharply, "Reagan's intention has been to maintain sole possession of the national government within the White House and nowhere else."

On one point, Lowi concedes, there is agreement between the president and himself. Where once big government was part of the solution, now it is part of the problem.

"I celebrate Reagan's recognition of the truth and regret the fact that he does not actually believe it. He embraced big government and embraced, nay, enlarged the plebiscitary presidency more than most of his predecessors."

Those citations, I believe, give a fair impression of the strengths and weaknesses of Professor Lowi's new book. He has a scholarly knowledge and penetrating understanding of American government. But his insights are sometimes dimmed by a peculiar poli-sci prose style, and his ability to persuade sometimes lessened by a certain cuteness.

The traditional American political system was centered in Congress and based on patronage. To its traditional function as the distributor of patronage, Lowi argues, Franklin D. Roosevelt added two new roles, which "profoundly and permanently changed the nature of the national government": regulation, and redistribution.

In my opinion Lowi concedes too little importance to the trends that were already predicting and preparing the modern presidency in the early 20th century. Yet he is clearly right that it was FDR who institutionalized and established the new, strong presidency, and that he did so by "pushing party to the periphery" and forging a new direct and unmediated relationship between the President and the people."

HE TRACES the new direct relationship between president and people through such phenomena as the growth of split-ticket voting and split electoral outcomes and the rise of independency. And he is surely right to see the use of public opinion polling as second only to the rise of television campaigning among the techniques that made the new presidency possible. He makes only passing reference to the fact that it is now all candidates for office, not only presidential candidates, who campaign independently of party.

One of Lowi's great gifts is the ability to condense a whole historical argument or analysis into a concise judgment, and here he gives us one of them. "FDR," he says, "tried to leave as a legacy a more relevant, programmatic party base for future presidents. In his failure, he left another kind of legacy. He left the beginnings of what was to become the plebiscitary presidency."

He should perhaps have spelled out more clearly what he means by that phrase. As I understand it, it refers to the concentration of national government in the hands of a president -- or a White House staff -- mandated by a more or less massive popular vote every four years, but in between times hardly conected with the people by intermediate institutions such as legislature or party. "Plebiscitary presidents," Lowi says, "have all the disadvantages of monarchy with none of the advantages."

One of the best sections of the book is devoted to the dangers of the plebiscitary presidency in foreign policy. Presidents must be stars. They conduct their own foreign policy. (Even Eisenhower, Fred I. Greenstein has shown, with the granitic John Foster Dulles as his secretary of state, controlled the grand lines of foreign policy more than used to be imagined; even Kissinger established his position as the president's man as against the State Department.)

They despise traditional diplomacy, its instinct for quiet accommodation, and seek dramatic solutions of problems that are not susceptible to solution. And, constantly, with numbing repetition, they oversell their doctrines and their imaginary victories.

Perhaps the most telling quotation Lowi makes is from an article by Hendrick Smith in The New York Times Magazine in September 1983, reporting a conversation with one of President Reagan's aides.

"We need a win somewhere," this statesman said, "whether it's in Central America, the Middle East or with the Russians." It is just as well for all of us that in the event it was in Grenada.

At home and abroad, the plebiscitary presidency has inflamed expectations, and failed to satisfy them. What can be done about it?

Like other analysts before him, Lowi is more convincing in his diagnosis than in his prescription. "Reform, to be successful," he rightly says, "would have to deal with the excessive personification of American government in the presidency." Yes, but how? By abolishing television, or reintroducing traditional party allegiance by statute?

Lowi is for "building down" the presidency, and one minimum suggestion he makes is that presidents, in what seem unlikely acts of self-abnegation, should veto legislation containing excessive delegation of power to the executive. A veto on the legislative veto?

He also demolishes quite convincingly the traditional arguments for a two-party system and argues, less convincingly, for a three- party system.

He is right that real reform of the malfunctions of American government will not come until there is real change in the points of view of powerful people. But he does not explain to my satisfaction how such change could take place in the minds of people who have become powerful precisely by exploiting the plebiscitary character of the system and do not believe there is anything that needs to be fixed as long as they are not broke.