Until recently, Jose Uribe was an obscure New Jersey businessman who had been caught up in what prosecutors say was a sprawling and lucrative bribery scheme involving Senator Robert Menendez and others.

But after Mr. Uribe pleaded guilty in March to trying to bribe Mr. Menendez and agreed to cooperate with the authorities, he vaulted into a more prominent position: star government witness.

On Friday, Mr. Uribe is expected to testify against Mr. Menendez, a New Jersey Democrat, in Federal District Court in Manhattan, prosecutors said, as the senator’s corruption trial ends its fourth week.

The senator and his wife, Nadine Menendez, are charged with conspiring to accept cash, gold bullion and other bribes collectively worth hundreds of thousands of dollars in exchange for Mr. Menendez’s agreement to direct aid to Egypt and to meddle in criminal cases in New Jersey. One of those cases involved Mr. Uribe.

Prosecutors say Mr. Uribe, a former insurance broker who worked in the trucking industry, sought the senator’s help to stave off criminal investigations the New Jersey attorney general’s office was conducting into two of Mr. Uribe’s associates. In return, an indictment says, Mr. Uribe helped to buy Ms. Menendez, then the senator’s girlfriend, a new Mercedes-Benz C-300 convertible worth more than $60,000.