DEMOCRATIC congressional leaders announced a formal probe of allegations that efforts were made to delay the release of Iranian-held US hostages to benefit the 1980 presidential campaign of Ronald Reagan.Calling the allegations "both persistent and disturbing," House Speaker Thomas Foley (D) of Washington and Senate Democratic leader George Mitchell of Maine said Aug. 5 committees from each chamber will undertake the probe. "We have no conclusive evidence of wrongdoing, but the seriousness of these allegations, and the weight of circumstantial information, compel an effort to establish the facts," Representative Foley and Senator Mitchell said in a joint statement. Rumors have been circulating that Reagan campaign operatives sought to negotiate a deal with Iran to delay the release of the 52 hostages to undermine President Carter's reelection bid. The allegations were revived earlier this year when Gary Sick, a National Security Council aide during the Carter administration, said his own efforts had uncovered circumstantial but persuasive indications that a deal had been engineered by Mr. Reagan's campaign chief, William Casey, who later went on to head the CIA. Reagan, President Bush, and officials of their campaign have repeatedly denied the allegations. Bush, who according to some reports was present at negotiating meetings in Europe in 1980, has described the allegations as "a vicious personal assault" on his integrity.