Why is it that the president of the United States, often called the most powerful person on earth, can't seem to get anything done?

Lyndon Johnson, who certainly undestood how to manipulate Congress better than any other modern president, used to grumble, "The only power I've got is nuclear . . . and I can't use that!" John Kennedy, who had a Democratic Congress, was once described as frantically snatching "for just enough power to get by the next day's problems." Richard Nixon, after a landslide victory, impounded at least $18 billion appropriated for programs he disliked: It appears to have been a pathetic attempt to get his way.

Godfrey Hodgson, British journalist, author and political pschoanalyst, has written what may well be a prescient book on the paradox of the presidency, "All Things to All Men." He discussed his ideas the other day on National Town Meeting with columnists Haynes Johnson and George Will, and National Public Radio's Linda Wertheimer ("Bring a conservative these days is almost as good as being a woman") moderating.

Quite simply, Hodgson believes our presidency doesn't work -- not because it needs more power, or less, not because of the personal dimensions of recent, current or putative occupants, but because the office has become dangerously isolated.

"The real problem is the president's connection with the rest of the system," said the 46-year-old veteran of several presidential campaigns. He outlined the modern presidency as "Invented by FDR from a blueprint by Teddy Roosevelt and Woodrow Wilson": leadership of Congress with program and agenda; creation of a big national coalition party; use of media to reach over power groups to contact the "people" themselves.

Today, however, Congress is fragmented by single-issue politics; the bureaucy has taken on a life of its own; the party system has deteriorated, and the Faustian power-vision offered by TV has soured, as one president after another finds there is such a thing as overexposure. The networks have been noticing this, too.

"Dramatic events like those in Iran and Afghanistan can put the president back in the spotlight for a time," he writes, "but the media's interest in the presidency seems increasingly tinged with cynicism. The public seems increasingly skeptical and indifferent . . . The modern presidency has come to depend too heavily on the media. It is not clear that the media can save it."

What to do?

One can see the problems more easily than the solutions, he said after the radio show. One things seems obvious: The surest road is not constitutional amendment and great monolithic reforms, but "gradual, practical, informal ways."

First, unfortuanately, things will probably have to get worse before they get better, until people see clearly that the system isn't working. "The Republicans say they want less government, but they'd better make sure the government that remains will work well. How exactly would Reagan cut taxes 30 percent without damaging its functions?"

The true conservative, he added, would do better to stop the eternal talk of less government and instead consider "how to give coherency to the government, how to protect the legitimacy and authority of the state . . . What is lacking here [in the attacks on big government] is a concept of the state, as distinct from the administration or the nation. What is the U.S. government? The FBI? The White House staff? The Joint Chiefs of Staff? We have no real sense of it."

Americans, Hodgson observed, have a healthy suspicion of government, and the Westerner's suspicion of the Eastern Establishment is also healthy: "The ambivalence about whether government really ought to be successful, the campaigning against Washington as a means of getting there." But at the same time, he said, we tend to overdo the insistence on absolute separation of powers before they can be balanced. "In practice, there just isn't absolute separation."

He suggested a permanent council of congressional leaders, the Speaker, the key committe chairmen, who would meet formally with the president, not to make commitments but to keep the two branches in touch with each other's thinking.

He also urged pruning the presidential appointments at the undersecretarial level and strengthening the White House staff with more professionals and fewer "program-oriented people" not interested in actually governing, people who are basically leftovers from the campaign with no place to go, people who in other times would have slipped back into the party woodwork.

And, oh yes, who will win? He thinks Reagan. But he doesn't believe the problems will budge an inch. Because the problems are institutional.