THE MODERN PRESIDENT is at once omnipotent and impotent, Godfrey Hodgson argues in this thoughtful and disturbing volume. Poised to exercise leadership, he finds an American constituency no longer schooled in followership. Elected to the most powerful office on earth, he strives to maintain an influence that will swiftly wane.

How to explain the paradox of the modern presidency? Hodgson points to the corroded links between the White House and the crucial sources of democratic authority. The president's chief weapon, Harry Truman believed, is the power to persuade. The targets of presidential persuasion today -- bureacucracy, Congress, party, media and, through them, the electorate -- are indifferent or obsolescent or intransigent.

Hodgson, a long-time and perceptive British observer of American politics, presents a convincing portrait of a presidency buffeted by the whirlwinds of latter-day Washington. As executive officials seek direction from private interest groups and congressional committees, the White House staff hectors Cabinet secretaries in the name of the president. "The best he can do," Hodgson observes, "is to rely on the hasty, hectic efforts of his 'small band of zealots' to ensure that the great, slow machine of bureaucracy, whose proper function is to administer the delivery of services to the citizens, delivers political booty to the President with which he can reward his followers and retain the loyalty of his 'disciple officials.'"

Congress has adopted the lineaments of the 18th-century Polish Diet, "where hundreds of independent members, bound by no ties of party and each as proud as a sovereign, jealously guarded the power to block anything the king might attempt, without ever organizing any means of putting forward alternative policies of their own." The deadlock ossifies as the American legislature assumes a greater share of responsibilities such as diplomacy and budget-making.

The political parties retain "little more than a vestigial function in presidential politics." Too frail to command siginificant support for presidential programs, they are mainly ignored for three years out of four. Among the excluded when Jimmy Carter called American leaders to Camp David in 1979, Hodgson reminds us, were the Democratic national chairman and a goodly share of the party's leadership in Congress.

Nor can press, radio and television any longer substitute as a source of influence. The modern chief executive finds that he must fence with a skeptical media establishment and a public that is tuning out. "An increasingly bored and cynical electorate is no longer so easily activated by appeals from the corner of the living room. The power of the magic is wearing off."

All of this is, for the most part, familiar territory, yet Hodgson presents it in a cogent fashion that serves him especially well in describing the stalemate between president and Congress. He has read widely and grounds his arguments on both contemporary journalism and scholarship. This balance is occasionally injured by an inclination to grand assertion. We are told, for instance, that "People supported George McGovern, after all, not because of what they could get out of it, and certainly not because they were overwhelmed by his charisma, but because they shared his convictions on the war."

The author tends also to overstate the direct influence of the media. "Television," he writes, "more than any other single force, has nominated and elected modern Presidents (all except Gerald Ford), established their authority, and kept them in power." Even the most jaded among us might resist the supposition that if Richard Nixon "had contrived to moderate the intensity of his administration's conflict with the media, then perhaps he would have gotten away with it and lasted out his second term."

Hodgson offers a prescription that is more tentative than his diagnosis. He asks Americans to lower their expectations of presidential performance and presidents to share their burdens more broadly. He believes that a national primary could increase participation and strengthen the parties. "If party leaders were to wheel and deal to preselect candidates ahead of the primary, the voters would always have the sanction of voting for the other party's candidate in the general election, or of 'going fishing.'"

He proposes that we more precisely distinguish the roles of our government institutions to reduce paralyzing duplications and competition. Yet he also suggests that the separation of powers doctrine be laid aside in favor of certain innovations, such as the legislative veto, that help to bridge the gulf between president and Congress without upsetting the balance of power. "It may be time to recognize that, while the value of a system of checks and balances has been proved over and over again, and not least in the Watergate crisis, such safeguards do not logically or necessarily demand an impenetrable separation between the executive and the legislature."

Hodgson believes that congressmen should serve four-year terms concurrent with the president's, committing all to a common ticket and platform, and that the president should be encouraged to appoint a kind of "supercabinet council" from among his party's congressional leaders. The members of this institutionalized colleagiality would frame presidential programs together and therefore be more notivated to fight for them together.

Although his book is being pulbished during a presidential campaign autumn, Hodgson has little confidence that the outcome will matter. The constraints upon our recent presidents, he maintains, have been more structural than personal. Would that Americans were as intrigued by the state of the presidency as by the selection of the president. For as this book cogently demonstrates, the powers of presidential persuasion today are in dangerous eclipse, and they can only be restored by thoughtful, daring, and unsentimental reform.