At a time when political commentators and editorial writers are decrying presidential debates that may turn more on TV personality than issues, the National Portrait Gallery held an all-day symposium yesterday on "Presidential Personality."

"The media say it is wrong to decide on personality over issues," said Marc Pachter, historiam of the National Portrait Gallery and moderator of the symposium. "But how a president responds to an issue depends, in part, on his personality."

Symposium speakers explored the characters of four 20th-century presidents -- Theodore Roosevelt, John F. Kennedy, Lyndon Baines Johnson and Richard M. Nixon. Two of the participants, Ted Sorenson and George Reedy, were close friends and colleagues of the presidents. The other two -- Edmund Morris, who won a Pulitzer Prize for "The Rise of Theodore Roosevelt," and Fawn Brodie, who is working on a "psycho-biography" of Nixon -- are biographers using traditional research tools.

"Kennedy was a strong president because he was a strong character," Sorenson told nearly 200 registrants attending the symposium.

And Morris noted that Teddy Roosevelt was a man with a "bigness of personality" and "seismic" effect despite his 5-foot-9-inch stature, size 7 shoes, and average, 42-inch chest.

Reedy, who served as press secretary to President Johnson, saw Johnson's accomplishments offset by the Vietnam war, but concluded: "Whether for good or bad, a giant. God, I wish we had one now." He saw Johnson in the role of the tragic hero driven to inevitable destruction.

Brodie was unable to attend because of illness, but her paper was read by Pachter, who noted that she has been a biographer of "driven men," including Thaddeus Stevens and Richard Burton. She detailed Nixon's history of lying and concluded that he was fascinated by "how close he can walk the edge." l

Sorenson, who was speechwriter for President Kennedy, said Kennedy fulfilled his role of national leader with eloquence that probably would be suitable for today's "cool" TV image and this more cynical, apathetic time. Kennedy, he added, would have thought the symposium on presidential personality was well-times, for he believed "one man could make a difference -- or should try" in contrast to the view that history is little effected by men but governed by broad, irreversible tides.