Winfrey joined Cory Booker in Jersey City for the Thursday fundraiser, where donors paid between $1,000 and $2,600 a head.
Meanwhile, in Newark where Booker is mayor, three candidates competing against Booker answered written questions from the audience about Social Security, Medicare and civil rights issues. Newark NAACP President Deborah Gregory said it would have been great if Booker had brought Winfrey to the forum.
Booker's Democratic opponents criticized him for raising money instead of answering questions. A Booker spokesman said the fundraiser was scheduled before the campaign was invited to the forum.
The candidates are running for the seat of the late Sen. Frank Lautenberg.