A Howard County Circuit Court judge ruled yesterday that legal arguments over whether prosecutors can use as evidence a tape of Linda R. Tripp's conversation with Monica S. Lewinsky will take place before the trial itself.

The first hearing in the case before Judge Diane O. Leasure was largely a preview of arguments expected Dec. 13 over the status of the pivotal piece of evidence in the case: the tape Tripp is charged with recording illegally.

Maryland prosecutors launched a pointed attack on the conduct of independent counsel Kenneth W. Starr's office during the early days of the presidential sex scandal, alleging that it failed to give Tripp proper legal protection, thus opening her to wiretapping charges. They argued that when she gave the tape to Starr's office Jan. 16, 1998, Tripp had only informal legal immunity, in the form of a letter from a Starr deputy promising protection from "any criminal case."

Starr's office did not have authority to offer her such broad protection, prosecutors said, and the tape therefore can be used against Tripp in court. They contended that Tripp's broad immunity kicked in Feb. 19, when a federal judge signed off on it.

The tape is the make-or-break issue: Without it, prosecutors conceded yesterday, their case would be weak.

"We would be in big trouble" if the judge ruled that Tripp was fully immunized Jan. 16, said Assistant Attorney General Carolyn H. Henneman, who argued on behalf of State Prosecutor Stephen Montanarelli.

Added Montanarelli, "It would be extremely difficult to continue," and he raised the possibility that he might withdraw the indictment against Tripp if he lost on this point.

Tripp's attorneys argued that the letter from Starr's office was sufficient to protect her from prosecution. Furthermore, they argued, even if Starr's deputies made promises they couldn't legally keep and Tripp's attorney failed to warn her of this, the fact that she thought she was fully protected is all that matters. Leasure, in one of her few interruptions during the two-hour hearing, appeared to have considered that line of reasoning.

Pressing Henneman on that point, she asked, "Isn't it what [Tripp] thought, not what counsel thought?"

The two sides now face a gantlet of legal proceedings. The judge ordered attorneys to try to reach agreement on the immunity issue by Thanksgiving Day. It is unlikely they will be able to agree, so both sides would begin arguing the issue before Leasure on Dec. 13 in a hearing that could last more than two weeks. Next would be a hearing on whether other evidence against Tripp could be used or the case should be thrown out.

The trial is scheduled to begin Jan. 18, though it is likely to be pushed into early spring. The hearing next month could feature testimony from Lewinsky, several current and former lawyers from Starr's staff and Tripp, lawyers close to the case said.

Tripp is charged with illegally taping a telephone conversation with Lewinsky, a former White House intern, on Dec. 22, 1997, and illegally disclosing its contents to Newsweek magazine. Each of the two charges carries a maximum of five years in prison and a $10,000 fine upon conviction. Tripp acknowledged before a grand jury last year that she made the Dec. 22 tape, despite knowing it was illegal to do so in Maryland, because she thought it could protect her from attacks by the White House.

Yesterday's hearing was the first time the 15-month-old case made its way into a public forum. It's been mostly a closed-door grand jury investigation, followed by a flurry of court filings. The two sides, which have aggressively attacked each other in court papers and the media, shook hands at the beginning of the 1 p.m. hearing.

The hearing focused on interpretations of the events that unfolded quickly around President Clinton's Jan. 17, 1998, deposition in the Paula Corbin Jones sexual-harassment case.

On Jan. 8, one of Starr's deputies learned that Tripp had the tapes. Shortly thereafter, he contacted her attorney, and they reached a verbal agreement that Tripp would hand them over in exchange for legal protection.

On Jan. 16, she received a letter from deputy independent counsel Jackie M. Bennett stating, "[N]o testimony or other information provided under this agreement, or any information directly or indirectly derived from such testimony or other information, may be used against Ms. Tripp in any criminal case."

The case against Tripp could come down to Leasure's interpretation of those words.

Tripp turned over the tapes Jan. 16 but wanted reassurance about her protections, her attorneys said. Starr's deputies obtained a court order from U.S. District Judge Norma Holloway Johnson on Feb. 19, formalizing the immunity agreement. It took close to a month to do so because Starr's deputies were consumed with the expanding investigation of the president, two lawyers close to Starr's office have said.

Prosecutors argued yesterday that Starr's prosecutors had the power to grant Tripp protection only from other federal prosecutors, not from Maryland prosecutors such as Montanarelli.

Starr's office "didn't have the authority to do that and bind us," said Henneman, who also said a prosecutor cannot grant blanket immunity "simply by referring to it in his letter," as Bennett did.

Tripp's attorneys disagreed, questioning not only the legality but also the fairness of the case against Tripp.

"The state cannot take advantage of a witness's cooperation and then turn around and prosecute them," said Joseph Murtha.