Hold On, Mr. President! by Sam Donaldson (Random House: $17.95; 294 pp.)

When word got out that Jimmy Carter had called Lyndon Johnson a liar in his notorious 1976 Playboy interview, candidate Carter had the misfortune to be campaigning in Johnson’s native state of Texas--and the equal misfortune to be covered by ABC television’s brash and intrepid Sam Donaldson.

For it was Donaldson who demolished Carter’s ill-conceived effort to imply that Playboy had misquoted him by collaring the candidate, reciting the offensive passage to his face and demanding: “Did you say that or not?” In that situation, as Donaldson notes in “Hold On, Mr. President!,” his memoir of his career as a television journalist, Carter had little choice but to fess up.

The zeal and skill displayed in that episode, and in other vignettes strewn through these pages, are characteristic of Donaldson’s work and have helped establish him as a top-notch White House correspondent and a dominant Washington media figure. He seems to think of himself as a sort of a people’s paladin, and his view of his work would have warmed the hearts of the framers of the First Amendment: “It’s a reporter’s job to challenge a President--every President--to explain and defend his policies whether you agree with them or not.”

The book provides plenty of evidence that Donaldson does his best to live up to that lofty goal--as indeed do many of his colleagues in print and electronic journalism. But they all have a big problem, which is also made clear in the book. For all their talent and verve, Donaldson and others like him in his trade are institutionally overmatched and ultimately ineffective in challenging the power and prestige of the presidency. In fact, I would argue that the combativeness sometimes displayed by Donaldson and other Washington reporters, particularly at presidential news conferences, works to the advantage of their adversary by creating for the President the illusion that he is seriously being held accountable.

Real accountability would require something like the question sessions held in the British Parliament, where the MPs who interrogate the prime minister are cloaked with the authority of the electorate. By contrast, as Donaldson shows, all the President’s journalistic inquisitors have on their side is the First Amendment, which entitles them to publish what the President chooses to say, but doesn’t compel him to answer their questions.

The author describes the all-too-familiar scene as the President heads toward his helicopter outside the White House while reporters hopefully fire their questions at him from a distance. “Ronald Reagan, smiling jovially, cups his hand to his ear, indicating that he can’t hear the question. He points to his watch as if late to an appointment. But if he gets a question he wants to answer his hearing improves dramatically, and he finds time, after all, to shout back a reply.”

Such behavior is demeaning not just to the press, but to every citizen. It is enough to make Donaldson ask, “What am I doing here and why am I doing it.” The answer he gives himself is “to find out what’s really going on at the White House.” But given the limitations under which the press works, that is almost impossible, a point illustrated by the current furor over the sale of weapons to Iran. As this is written, it has been about three months since that story broke --significantly enough, in a Beirut magazine--and though the press corps has diligently pursued this tangled tale, the country had to wait for The Tower Commission Report to get its first inkling of what “really” went on at the White House.

No one can accuse Donaldson of taking himself too seriously. Asked by his network bosses how much time he would need to write this book, he answered: “Every Thursday for two months.” Judging from his rambling, disjointed presentation, that is about all the time he took. Still, along with a mishmash of gossip and trivia, Donaldson does remind us of the sobering realities governing American politics and journalism.

“Look, I won,” Reagan replied after his landslide reelection when asked if he would hold more press conferences in his second term. “I don’t have to submit myself to. . . . “

“At this point everyone laughed,” Donaldson writes. “But the record shows he meant it.”

Indeed he did. And that in itself is nothing for any of us to laugh about.