Gerald Rafshoon, an Atlanta advertising executive who has long been an unofficial member of President Carter's inner circle of advisers, will join the White House senior staff July 1 as assistant to the president for communications, White House press secretary Jody Powell said yesterday.

Rafshoon, who directed Carter's television advertising during the 1975 presidential campaign, will take over some of Powell's duties, including supervision of the White House television operation, Carter's speechwriters and the press advance office.

But, according to Powell, Rafshoon's chief responsibility will be to plan and execute long-range public relations efforts to explain the president's programs to the public.

Powell has candidly conceded that this has been a major failing of the Carter administration. He told reporters recently that he had determined he could not perform such a function as well as serve as press secretary and that he planned to create a senior White House position to fill that gap.

The choice of Rafshoon for the job, however, came as something of a surprise. Powell's determination to improve the work of the White House press office is part of a general reshuffling of the White House staff aimed in part at broadening Carter's circle of closest advisers with the addition of experienced outsiders.

But Rafshoon is the epitome of a Carter insider. He has known the president and his top aides for years and, along with political polster Pat Caddell, has been an unofficial member of the White House staff since the first day of the administration.

The clearest loser in the addition of Rafshoon to the senior staff is Barry Jagoda, the president's television adviser. Jagoda will now report to Rafshoon, whose main expertise is in television advertising, rather than Powell.

Since Carter's election, Rafshoon has opened an office of his advertising agency in Washington and spent much of his time here. Powell said Rafshoon will be expected to meet the same financial disclosure and conflict-of-interest standards as other senior aides and to insulate himself from his advertising business.

Rafshoon's salary at the White House will be $56,000 a year. He is the 10th aide to be named to a position at the salary. If Mayor Richard Hatcher of Gary, Ind., accepts an offer to work at the White House, the senior staff will number 11. Carter began his term with a senior staff of seven aides.