A bureaucratic snag in the Department of Justice has imperiled its prosecution of political extremist Lyndon H. LaRouche Jr. and 13 associates, whose trial in Boston on credit card fraud and obstruction of justice charges is scheduled to start Monday, law enforcement sources said.

Prosecutors had planned for some time to obtain grants of immunity for numerous unindicted LaRouche followers to compel their testimony, which is considered to be key. But under federal law, the only person who can grant immunity is a U.S. attorney, and Boston has only an acting U.S. attorney, Frank McNamara, whose appointment has not been acted upon by the Senate Judiciary Committee.

The Justice Department could remedy the situation by naming an interim U.S. attorney, who temporarily would have power to grant the needed immunity, sources said. But sources said that Attorney General Edwin Meese III and top aides are refusing to take action because they are trying to show what one official called a "macho" face to the Democratic-dominated Senate.

A grant of immunity makes a witness immune from prosecution based on the information he gives to authorities.

Relations already were a bit frosty between top Justice officials and some members of the Senate Judiciary Committee over this week's confirmation hearings for Judge Robert H. Bork to the Supreme Court.

Law enforcement officials are expressing frustration with the situation, with one saying it is "awful" that the stalemate could damage the prosecution of LaRouche and his associates. Dozens of FBI agents have been working on the case for 23 months, and Justice Department officials once called it a top priority.

Assistant U.S. Attorney John Markham, who is handling the case in Boston, explained the stakes to U.S. District Judge Robert E. Keeton in court yesterday, and asked him to rule quickly on whether McNamara could have the power to grant immunity despite his acting status.

Markham told Keeton he would be presenting "an extremely different case" against the LaRouche group if he is prevented from obtaining immunity for a number of LaRouche associates. Markham said it is "extremely important" that Keeton resolve the matter before Markham makes his opening arguments, estimated to take place in about 10 days.

LaRouche group attorneys have already said that they will move to stop grants of immunity to any LaRouche associates who are subpoenaed. The situation also stymies a number of other federal probes there, including one into Boston municipal corruption, sources said.

McNamara could not be reached for comment yesterday.

Justice spokesman John Russell declined comment on the stalemate, but said that McNamara, as an acting U.S. attorney, "has the power to ask for grants of immunity, and we're prepared to argue that in court." Last month Russell told a Boston newspaper that "unless this thing is resolved, the entire grand jury system up there could be held in abeyance . . . . We're facing a constitutional crisis over this thing."

McNamara's main allies are conservative aides to Meese, who are drawn to McNamara in part because of his conservatism. McNamara, who has little experience in federal court and none as a prosecutor, was a founder of Citizens for America, a conservative lobbying group set up by former New York gubernatorial candidate Lewis E. Lehrman. In 1982 he ran for Congress against then-House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

Meese named McNamara interim U.S. attorney in April, and President Reagan sent his nomination to the Senate then.

Meanwhile, McNamara was late in getting to the committee responses to FBI questions about his background. The committee has been probing his 1982 campaign finances. Because of the lateness, the committee has not acted further on the nomination.

Top Meese aides see the committee's action as a slight, and are refusing to appoint another interim U.S. attorney for purposes of granting immunity.

Under federal law, McNamara's "interim" appointment lasted only 120 days, and expired Aug. 4. After that date -- when Meese named him acting U.S. attorney and his power to grant immunities came into question -- the Boston judges could have decided as a body to name McNamara to the vacancy pending Senate action. But they declined.