With the public phase of the congressional hearings on the Iran-contra affair winding down, the next move is likely to come from independent counsel Lawrence E. Walsh, who for eight months has been quietly gathering evidence for possible criminal charges.

Since his appointment last December, Walsh has been close-mouthed about where he is headed.

However, congressional testimony, court proceedings and interviews with defense attorneys and other sources indicate that Walsh is pursuing a broad case alleging conspiracy to defraud the government. Specific charges under consideration range from obstruction of justice (stemming from destruction of White House documents) to misuse of government funds.

So far, the central figures in Walsh's investigation are believed to be Marine Lt. Col. Oliver L. North, the fired National Security Council aide, and Rear Adm. John M. Poindexter, former national security adviser.

In addition, Walsh has been closely examining the private Iran-contra network of former military and intelligence officials headed by retired Air Force major general Richard V. Secord and his business partner, Albert A. Hakim, according to informed sources. Several mid- to low-level Secord associates have received immunity from prosecution, sources said.

Richard W. Beckler, Poindexter's lawyer, went so far as to announce at the hearings that Poindexter was a "primary target" of the federal grand jury investigating the Iran-contra affair and that the admiral faced the "imminent threat of prosecution."

Brendan V. Sullivan Jr., North's attorney, did not specify at the hearings whether his client is a target, but he did say that Walsh is "looking at Col. North and everything he did."

"I have never been in a position where a client is forced to testify about all matters which are the subject of a pending indictment," Sullivan said at the outset of North's testimony. North was the only person cited by name in the order appointing Walsh.

North and Poindexter as well as several other witnesses were compelled to testify before the House and Senate panels after the committees granted them limited immunity from prosecution. The immunity grants prevent Walsh from using North's and Poindexter's testimony in a possible criminal case, but Walsh and his aides have been busy seeking to develop evidence separate from their congressional statements.

Walsh has routinely been filing sealed reports in U.S. District Court detailing his evidence, anticipating challenges that the prosecutor's investigation has been tainted by immunized congressional testimony.

With only the outlines of Walsh's investigation of former administration officials emerging, it is difficult to gauge whether anyone currently in the administration might be under scrutiny.

Assistant Secretary of State Elliott Abrams, the controversial administration point man on the Nicaraguan contra program, told reporters this week that Walsh's office has informed his attorney that he is not a target of the investigation.

Sources have previously said that Walsh is examining the role of Attorney General Edwin Meese III in federal investigations related to North's secret operations, especially involving the contras. Meese has defended his actions as proper.

Meanwhile, the Senate and House Iran-contra panels continued their work yesterday, hearing testimony in executive session from two Central Intelligence Agency officials, Duane R. (Dewey) Clarridge and Alan D. Fiers. Clarridge and Fiers were both involved in the secret contra war, and Clarridge also played a key role in a November 1985 Israeli shipment of U.S. arms to Iran. Clarridge and Fiers did not testify under any grants of immunity.

Long after the Iran-contra committees issue their report, the prosecutor's activities seem likely to serve as a source of continuing information about the Reagan administration's biggest political crisis.

Walsh already has obtained convictions of two men involved in private contra fund-raising efforts that were encouraged by the White House.

Carl R. (Spitz) Channell, a conservative fund-raiser, and Richard R. Miller, a public relations executive and former Reagan campaign aide, pleaded guilty to a charge of conspiring to defraud the government by using tax-deductible contributions for contra military assistance.

Channell and Miller agreed to cooperate with Walsh's investigation and both named North in court as a co-conspirator.

President Reagan has acknowledged speaking to contributors to Channell's operation, but he said he thought the money was going for television advertising, not military purposes.

White House officials stress that Reagan was not involved in the Channell-Miller criminal conspiracy, but have expressed concern that Reagan may have unwittingly aided them.

Some Republican members of the Senate and House panels have criticized Walsh for pursuing a broad conspiracy case.

Shortly before the hearings began, Sen. Warren B. Rudman (R-N.H.), vice chairman of the Senate panel, said that a conspiracy case would be difficult to prove in court and that by following such a strategy, Walsh was unnecessarily prolonging his inquiry. "There are too many important issues facing the country for the American people to wait while {Walsh's} investigation goes on ad nauseam," Rudman said.

Rudman, a lawyer and former attorney general in New Hampshire, said that Walsh would be smarter to confine his investigation to specific charges such as obstruction of justice.

Other administration supporters have argued that criminal charges are not warranted in the Iran-contra affair because it is the story of a failed policy, not a criminal conspiracy. The Iran-contra operations, they say, were carried out by people who believed they were serving the president.