Persistent but unproven accusations that Ronald Reagan's 1980 presidential campaign negotiated a secret deal with Iran to prevent the release of American hostages until after the election are being revived this week with fresh accounts of meetings between campaign officials and an Iranian cleric. One of the accounts is provided by Gary Sick, a Middle East specialist who helped handle the Iranian hostage crisis as a member of the White House staff in the Carter Administration. Mr. Sick ... has heard what he considers to be reliable reports that a secret deal involving the hostages was begun during two meetings between William J. Casey and the Iranian cleric in a Madrid hotel in July 1980. The allegation that there were meetings between Mr. Casey, Mr. Reagan's campaign chairman, who went on be the Director of Central Intelligence, and Hojatolislam Mehdi Karrubi, a representative of Ayatollah Ruhollah Khomeini, has been reported for the first time by Mr. Sick. The fate of the hostages was a pivotal issue in the 1980 election. They were taken prisoner when followers of Ayatollah Ruhollah Khomeini ... seized the United States Embassy in Teheran in November 1979. A military operation to rescue them failed in the Iranian desert in April 1980. The Carter Administration hoped that it might obtain their release either through negotiations or a second rescue mission before Election Day, and Reagan campaign officials were concerned that the return of the hostages could swing the election to Mr. Carter.