Clark Clifford, lawyer, sage and counselor to presidents, is a pooh-bah of Washington's establishment. And since Ronald Reagan has gone out of his way to make the capital's establishment respectable again, this is an especially good time to visit with Clifford -- which "Bill Moyers' Journal" does tonight (Channel 26 at 9) in the first of two one-hour sessions.

The subject is the presidency, and while Clifford's views are mired in the mainstream -- "Americans want presidents to be better than they are," he declares -- they can be entertaining when he gets to personalities.

Harry Truman, Clifford's first boss, was, he says, "decisive, courageous and modest." He also had a passion for eight-handed poker with folks like Averell Harriman, Lyndon Johnson and the chief justice. Dwight D. Eisenhower was just the right man for his time and would have gone on getting elected until he died, Clifford believes, if the Constitution permitted.

The word for John F. Kennedy, says Clifford, was "grace." Johnson as president was "raw power . . . he reveled in the job every minute he had it; in good times and bad." Richard Nixon would have been a successful president, Clifford wryly notes, "if he believed in our system . . . he didn't believe in the First . . . Fourth . . . and 10th Amendments."

Gerald R. Ford, in Clifford's view, was the "great healer," a president who exactly fitted the country's mood of the moment.

Then came Jimmy Carter, and perhaps because his tenure was so recent, Clifford's assessment of him is more interesting than that of the other men. He was, says Clifford, an "enigma . . . honest, able, intelligent, industrious . . . yet the existence of blocks kept him from being successful." The essence of these, Clifford contends, was that Carter wasn't political enough somehow, unable to reach out to Congress, the press and the establishment (although Clifford himself never uses the dreaded word).

Carter wanted to stop what Clifford calls "the Washington game . . . but life doesn't work that way." Even Abe Lincoln, he notes, was a pol. Jimmy Carter was not, or at least not enough.

Take the Panama Canal. The time to give it back, according to Clifford, was in the third year of Carter's second term" when he could afford to be a statesman" and not at the start of his first, guaranteeing the lasting enmity of a substantial portion of the electorate.

Now comes Ronald Reagan. Clifford thinks the United States is still "fundamentally a liberal country" committed to the social programs that have developed since the Depression but united on the need to combat inflation. On foreign policy, Clifford is worried about the bombast of Reagan's rhetoric on the Soviet Union.

Moyers, as always, is engaging and warm. He likes Clifford (they were both close to Lyndon Johnson) and if you are, as many of us tend to be, an aficionado of Washington types, you'll probably like Clifford too.