President Carter's melancholy performance during his prime-time press conference July 20 helps explain why Gerald Rafshoon, his senior image rebuilder of six weeks' standing, is called "the PR Potemkin" within the White House.

That evening's follies were all under Rafshoon's stage management: the first prime-time news conference by this president, the ban against an opening statement praising the Bonn economic summit, the decision to refuse questioning on the Peter Bourne affair. Since joining the White House staff full time, Rafshoon has failed to slow the president's long decline.

Probably nobody could. What brings grumbles in the White House - no longer feud-free - is that Atlanta advertising man Rafshoon has not used his personal influence to guide the president into a more orderly course. Colleagues complain that Rafshoon's greatest accomplishment so far has been his own aggrandizement as "the curly-haired media whizz,"

His first initiative upon entering the White House chilled expectations that a sophisticated public-relations operator had come aboard. He proposed a Fourth of July nationally televised speech by Carter based on a patriotic theme, actually to be delivered on June 29 before the president started his vacation.

Cautious suggesting that that would not help the president much did not move Rafshoon. Nor did he approve a draft speech prepared by a staffer. Nothing would do but that the speech must be written by the eminent liberal historian, 75-year-old Henry Steele Commager. Reached in Denmark, Commager set to work quickly on a long speech. It was soon pouring into the White House, page by page, on the diplomatic cable from the Copehagen embassy.

Commager now assumes the idea was discarded. In fact, the networks said no to free television time. From that moment, suspicion was planted at the White House that Rafshoon more closely resembled the naive inexperience of the past 18 months than its antidote.

While planning the aborted Fourth of July operation, Rafshoon was in charge of a major political speech by the president to be given in Houston on June 23. Although Rafshoon promised a "thematic" approach in future Carter speeches, the energy theme originally for Houston was lost in a typically Carterite ramble. Whether Rafshoon really tried or not, his impact was imperceptible.

But Rafshoon clearly views the speech-writing operation as the core of his new job. He has tangled with senior political aide Anne Wexler, who has strong views about political input, over speech-writing responsibility. "I think we have some very talented speech-writers who need some direction," Rafshoon told us. Whether or not those speechwriters need that help, Rafshoon's prinicipal role as an insider is to get the president to read and digest in advance what is written for him.

Thus, the rambling Houston speech doused early hopes. Rafshoon seems less interested in the words and substance that make up speeches than in image-making. Short of getting the President to go through speech drafts prepared for him, Rafshoon himself is not keen about wading through printed words.

It is particularly galling for some Carter aides that, while doing little to rehabilitate the president, Rafshoon has become a darling of the media. "Sightseeing to the drumbeat of image-builder Gerald Rafshoon," began the New York Daily News in explaining the president's July 6 tours of Civil War battlefields. "Carter's media wizzard" was reported as telling the president, "in effect, to go out and meet people."

Rafshoon is probably more victim than perpetrator of such publicity. Particularly anxious of late to halt the Rafshoon media blitz, he rejected a recent bid from ABC's "Issues and Answers" and vetoed a proposed article on him for people magazine. "Jerry has turned down about three-fourths of the media requests," reports a sympathetic colleague, "but he probably should have turned down the other fourth, too."

Rafshoon complains that every public movement by the president is unfairly attributed to his advice. He denies parentage of either the Civil War battlefield tour or the president's much-publicized warning of dismissal for illegal-drug users on the White House staff. Nevertheless, staffers profess to see Rafshoon's imprint on the president's anti-drug memorandum (considering the vigor with which Rafshoon personally threatened his own subordinates if they smoke marijuana).

Whether or not Rafshoon is personally responsible, the release to the press of Carter's "confidential" memorandum against staff drug use is all too reminiscent of the fumbling amateurism that has plagued his White House. Rafshoon's failure thus far to introduce professionalism - while being so lavishly publicized as Jimmy Carter's savior - explains why uncharitable colleagues compare him to Prince Potemkin