A former Internet entrepreneur who could be a key witness in the federal criminal trial of a top finance official with Senator Clinton’s 2000 campaign has entered into a plea deal with the government in a separate case, according to persons familiar with the matter.

Peter Paul, 56, is expected to enter a guilty plea on one felony count of stock manipulation, during a hearing tomorrow in federal court at Islip before Judge Leonard Wexler.

In an interview yesterday, Paul’s lawyer, Joseph Conway, declined to discuss the agreement in detail. “It’s a bunch of different factors that played into it,” Mr. Conway said. “It’s a question of, based on the evidence, do you go to trial or work out a deal.”

Paul was indicted in June 2001 on stock fraud and conspiracy charges following the collapse of Stan Lee Media, a publicly traded company that planned projects for the Web, television, and films based on cartoon characters. Investigators charged that Paul, who was a founder of the company, operated a “pump and dump” scheme to artificially inflate the price of Stan Lee Media stock, while he was secretly selling his own shares.

Paul has contended that he was acting to protect the company’s shareholders from other individuals who were attempting to force the price down.

A spokesman for prosecutors handling the case against Paul, Robert Nardoza, declined to comment for this story.

The resolution of the stock charges against Paul could clear the way for him to testify at the trial of the national finance director of Mrs. Clinton’s 2000 campaign, David Rosen. In an indictment unsealed in January at Los Angeles, Mr. Rosen was charged with four felony counts of causing false statements to be submitted to the Federal Election Commission. The reports pertained to a star-studded August 12, 2000, fund-raising event that Paul helped to organize and bankroll. Mr. Rosen has pleaded not guilty to the charges.

In court filings, the government claimed that the omission from the reports of hundreds of thousands of dollars of in-kind donations was part of a scheme to maximize the amount of so-called hard money that Mrs. Clinton could spend. Under campaign finance laws at the time, hard money was more valuable to candidates because such funds could be spent on a broader range of campaign activities.

Asked about the likely plea, Mr. Rosen’s attorney, Paul Mark Sandler, declined to comment. “It’s inappropriate,” he said in an interview. “We will confine our comments on the facts and issues to the courtroom.”

A judge at Los Angeles was scheduled to hear arguments today on several motions Mr. Sandler has filed to dismiss the charges against his client. However, that hearing was postponed until March 15.

Paul is also suing President Clinton and Mrs. Clinton for fraud in a California court. He alleges that he made donations of about $2 million to the Senate campaign as part of a broader effort to get Mr. Clinton to join the board of Stan Lee Media after leaving office. The lawsuit claims the Clintons and their allies failed to report Paul’s donations and broke promises they made about the president’s plans to work for the company. The Clintons have sought to have the civil case thrown out, but so far those efforts have met with little success. Last October, the California Supreme Court declined to hear one appeal. Another is pending before a California appeals court.

The president of a conservative legal group that has given legal assistance to Paul refused yesterday to discuss the expected plea. “Peter’s position is he’s always wanted to hold everyone accountable. He’s been hopeful that the Clintons will be held accountable,” said Thomas Fitton of Judicial Watch.

Mr. Fitton predicted that whatever Paul does tomorrow, the Clintons are facing problems in both the civil case and the prosecution of Mrs. Clinton’s former finance chief, Mr. Rosen.

“It’s going to be a tough year legally for the Clintons, no matter what happens on Tuesday,” Mr. Fitton said. “The civil case, it’s just a matter of time before it goes full speed ahead. Mrs. Clinton’s appeal is desperate.”

The Justice Department has indicated that its investigation into the fundraising for Mrs. Clinton’s campaign has concluded and that no more indictments are expected. The Federal Election Commission is also conducting its own investigation into the matter.

An attorney for the Clintons, David Kendall, declined to comment yesterday on Paul’s apparent decision to plead guilty.

Paul’s involvement in the August 12, 2000, fund-raiser became an embarrassment for Mrs. Clinton after it was reported that he had three felony convictions. In 1979, Paul was convicted of cocaine trafficking and for his participation in a bizarre coffee-trading scheme that defrauded the Cuban government out of nearly $9 million. His license to practice law was suspended. Paul later pleaded guilty to a charge that in 1983, while returning from Canada, he showed false identification to a customs inspector.

Mrs. Clinton’s campaign returned a $2,000 donation from Paul. However, he contends he made at least $1.9 million in in-kind gifts for the fund-raiser, which took place in the Brentwood Hills of Los Angeles and featured celebrities such as Cher, Paul Anka, and Dylan McDermott.

As Stan Lee Media ran aground at the end of 2000, Paul flew to Brazil. American prosecutors initiated extradition proceedings, during which Paul spent about two years in a Brazilian prison.

Paul has said that his trip to Brazil was not prompted by possible charges in connection with the demise of Stan Lee Media. He has maintained that he traveled to Brazil to tend to a language business he owned there, Mondo English.

Paul ultimately lost the extradition fight and was returned to America in September 2003. He was kept behind bars until January, when he was released on a $1.2 million bond.

The stock manipulation charge to which Paul is expected to plead guilty carries a possible sentence of up to 10 years. Paul’s lawyer, Mr. Conway, said prosecutors have made no promises about what sentence they will seek. “There’s no guarantees. It will be up to the judge at the end of the day.”

However, Mr. Conway said he does not expect Paul to be sentenced for many months. “I think sentencing will be way down the road,” the attorney said.

That schedule would permit Paul to testify at Mr. Rosen’s trial. Mr. Conway could then ask Judge Wexler to take Paul’s assistance in the Rosen case into account. “We can make the direct pitch to the judge that the cooperation is worth something – to give him a lower sentence,” Mr. Conway said.

Paul’s case is pending in Islip because another defendant in the securities fraud case, Jeffrey Pittsburg, ran a brokerage firm on Long Island, Pittsburg Institutional. He has pleaded not guilty.

Another defendant and a former executive vice president of Stan Lee Media, Stephen Gordon, offered a guilty plea last month to a charge of stock manipulation.

The veteran comic book designer Stan Lee, who created such characters as the Incredible Hulk and Spider-Man, was also a principal at the now-defunct Internet company. He has not been charged with any crime.