A key staff member has resigned from the presidential commission investigating the Three Mile Island nuclear power plant accident. He cited "personal considerations" in discussing the resignation yesterday.

Ronald Natalie, chief counsel to the 15-member coimmission since President Carter set it up in mid-April, said in an interview he left because he had been "functioning as an adminstrator rather than as a lawyer."

At the same time, the commission announced the postponement until mid-July of public hearings that had been scheduled today through Friday in order to "further discuss issuing subpoenas for witnesses and documents."

Natalie said in his June 1 resignation letter to the commission chairman, John G. Kemeny, that he had made his decision "reluctantly and with deep regret." He was leaving durign the preliminary stages of the probe, he said, to "avoid any inference that my departure is associated with differences in principle" at the time conclusions are being drawn later on.

Sources on Capitol Hill and observers within the nuclear power community said the commission ahs had difficulties in organizing and actual investigation of Three Mile Island rather than merely conducting a rehash of what others have already learned. The pressure of a six-month deadline is already being noticed, as one of these sources put it, in "a general feeling of unhappiness that they are not getting to the bottom of things."

Natalie stressed that there had been no serious personal friction between him and the commission. "I was issuing instructions and organizing meetings, running a large staff of lawyers rather than lawyering myself," he said.

"I am satisfied that the investigation is going forward with complete vigor, as fast as it should or can," Natalie continued. "If anything, it can be criticized for moving too fast."

Kemeny said in a formal statement earlier he was "extremely sorry" Natalie had resigned. Natalie will return to the Washington law firm of Verner, Liipfert, Bernhard and McPherson, and will be replaced by his deputy, Stanley M. Gorinson.