The cooperation agreement reached between prosecutors and an employee of the call-girl ring known as Emperors Club VIP will come in handy should the Manhattan U.S. attorney, Michael Garcia, decide to charge Governor Spitzer with a crime.

The woman, Temeka Rachelle Lewis, 32, who entered a guilty plea to money laundering and prostitution-related charges yesterday, would be a key witness against Mr. Spitzer if the former governor is charged in connection with patronizing an Emperors Club prostitute, legal experts say.

Lewis, who booked clients for the call-girl service, had several phone conversations with Mr. Spitzer to hammer out the logistics of payment and scheduling for his tryst with a prostitute in Washington, D.C., on the eve of Valentine’s Day. Mr. Spitzer resigned days after details of those conversations were included in court papers charging four people with running Emperors Club.

“If they were going to prosecute him, it would be simpler and would present a more compelling case at trial to present her as a witness,” a professor at Fordham University School of Law, Ian Weinstein, said.

Lewis’s role in Emperors Club involved contact with clients, prostitutes, and the ring’s organizers. She, among the four charged, is presumed to have the most knowledge of the regularity of Mr. Spitzer’s trysts, how he paid, and his preferences, a source with knowledge of the case, who requested anonymity, said.

The text of Lewis’s cooperation agreement is largely standard in that it requires her to testify at any grand jury or criminal trial at the request of the U.S. attorney’s office. The agreement does contain one unusual clause that indicates that prosecutors are trying hard to keep secret any developments arising in their investigation of Mr. Spitzer. Lewis is required to notify prosecutors before discussing her knowledge of Emperors Club with anyone other than her lawyer or law enforcement officials, according to the agreement.

The U.S. attorney’s office has given no clear sign about whether it will prosecute Mr. Spitzer. Legal experts say Mr. Spitzer violated federal law when he asked Lewis to send him a prostitute from New York to meet him in Washington. Mr. Spitzer’s lawyers have not yet spoken publicly about the case.

There is also the possibility that prosecutors use Lewis’s testimony only against the three other defendants accused of running Emperors Club.

“The most I would say is that it is consistent with the government planning additional prosecutions,” Mr. Weinstein said of yesterday’s guilty plea. “It’s not clear to me there’s anything distinctive about what they have done to offer any signal that additional prosecutions will include Governor Spitzer.”

Lawyers involved in the case say they expect guilty pleas in the next few weeks from two other people whom prosecutors say are linked to Emperors Club: the alleged leader, Mark Brener, and a woman who is accused of working on a part-time basis in a similar capacity to Lewis, Tanya Hollander.

Lewis, who graduated with an English degree from the University of Virginia, said little yesterday beyond pleading guilty and admitting that she worked for Emperors Club. Her mother and twin sister were up from Virginia to be present for her guilty plea at U.S. District Court in Manhattan. Although the charges permit a judge to sentence her to as much as 25 years in prison, federal guidelines suggest she will be released after less than two years.

“She’s basically a very good person,” her lawyer, Marc Agnifilo, told reporters afterwards. “Sitting at the defense table in a federal courthouse is the last place she imagined she would be.”