Jack Abramoff, the Republican lobbyist under indictment for fraud in South Florida, is expected to complete a plea agreement in the Miami criminal case, setting the stage for him to become a crucial witness in a broad federal corruption investigation, people with direct knowledge of the case said.

One participant in the case said the deal could be made final as early as next week.

The terms of the plea deal have not been completed, and the negotiations are especially complicated because they involve prosecutors both in Miami and in Washington, where Mr. Abramoff is being investigated in a separate influence-peddling inquiry, participants said. Details of what he feels comfortable pleading guilty to are "probably largely worked out," the participant said, while the details of the prison sentence are less resolved.