The biggest political donor in New Jersey pleaded guilty on Wednesday to trying to compromise a witness in a campaign finance investigation by setting him up with a prostitute and secretly videotaping the sexual encounter. The plea came in a case that the federal prosecutor called a clear example of corruption in the state's political culture.

The donor, Charles Kushner, a real estate developer who has contributed millions of dollars to Democrats, is expected to serve 18 to 24 months in prison under his agreement with the United States attorney's office. He pleaded guilty to filing false tax returns and campaign finance reports in addition to one count of retaliating against a witness.

Mr. Kushner's plea agreement, however, does not include cooperating with federal investigators in the underlying campaign finance investigation or in any others, including an inquiry involving former aides to Gov. James E. McGreevey. Mr. Kushner and businesses he controls gave more than $1.5 million to Mr. McGreevey and his campaign committees, and Mr. Kushner had business dealings with the governor's former campaign manager and chief of staff, Gary Taffet, now the subject of a federal investigation.

Mr. Kushner was also nominated by the governor to lead the Port Authority of New York and New Jersey, although he withdrew when some legislators raised objections.