THE nomination of Robert Gates for director of the Central Intelligence Agency is in trouble.Mr. Gates's prospects took a turn for the worse Tuesday when former top CIA official Alan Fiers Jr. revealed that he and other senior intelligence officials knew that the proceeds from arms sales to Iran were secretly diverted to the Nicaraguan contras before the scandal was publicly revealed in November 1986. Sen. David L. Boren (D) of Oklahoma, chairman of the Senate Intelligence Committee, yesterday delayed confirmation hearings scheduled for Monday until the committee has had enough time to review the new information. Mr. Fiers pleaded guilty to two misdemeanors of illegally withholding information from Congress, and has agreed to cooperate with special prosecutor Lawrence Walsh. Other senior Reagan administration officials may also be prosecuted, including former State Department Latin American affairs director Elliott Abrams and Donald Gregg, now US ambassador to South Korea. Gates, who at the time was deputy director of intelligence and now serves Bush as deputy national security adviser, has not been under investigation himself. But now that Fiers has fingered both Gates's immediate superior at the time, the late director of central intelligence (DCI) William Casey, and his immediate subordinate, deputy director of operations Clair George, the burden lies on Gates's shoulders to prove that he was, as he maintains, ignorant of the scheme. "I'm afraid [Gates's] nomination is sinking fast," says a former senior CIA official who spoke on background. The official, who held a senior CIA post during the Iran-contra period, says it is plausible that Gates did not have "substantive knowledge" of the deal. "Casey ran things through his back pocket and he wouldn't have used an analyst [such as Gates] for operations," says the official, who feels Gates would make an excellent DCI given his experience both as a "producer" of intelligence at CIA and a "consumer" at the White House. He lauds Gates's desire to make intelligence "useful not "so caveated as to be useless," as Gen. Norman Schwarzkopf complained in congressional testimony after the Iraq war. But the official also acknowledges that he himself knew what was happening at the time, even though he wasn't supposed to, and that it would be tough for Gates to convince the Senate committee that he really was out of the loop. On Wednesday, President Bush sought to combat the latest blow to his nominee by issuing effusive praise at a press conference. He said he had told Gates that morning, "You're my man. I'm all for you. And don't let them get you down." But senators aren't known for taking risks, and the Fiers plea makes their dilemma tougher. The concern is that they will look bad if they confirm Gates and something is revealed later that links him conclusively to Iran-contra. When Bush nominated Gates on May 14 to replace retiring DCI William Webster, the general feeling in the Senate was that Gates would be confirmed, albeit with some reservations. Before Gates was nominated, the White House took a careful pulse of the Senate's view of him and concluded he would pass. But even before the Fiers plea bargain, doubts about Gates's link to Iran-contra proved to linger longer than administration officials had thought they would. Four years ago, Gates had to withdraw his first nomination to become director of central intelligence in the face of questions over the same scandal. Since then, Gates had improved his image by his performance in the White House as deputy national security adviser, especially as one of Bush's key advisers during the battle against Iraq. But since his second nomination last May, the Senate has appeared to stall in actually starting confirmation hearings. The intelligence committee asked Gates to answer in writing more than 80 questions in preparation for the hearing. His responses were returned to the committee just before the July 4 recess. Now that Fiers has agreed to cooperate with independent counsel Walsh, more revelations and indictments are expected. Though other top Reagan administration officials appear set to take a fall for their involvement in the illegal funding of the contras, President Bush, who was vice president at the time, is not in trouble, says the ex-senior CIA official who spoke on background. The former official also dismissed questions about Gates that have come up over other matters, including: reported CIA knowledge of illegal arms shipments to South Africa and to Iraq; and a possible role in the so-called October surprise, in which the 1980 Reagan-Bush campaign allegedly cut a deal with Iran to ship them arms in return for delaying the release of the hostages being held in Tehran until after Ronald Reagan was elected. The ex-official called allegations of all these undertakings untrue.