LITTLE ROCK, Ark. -- A figure from the Whitewater investigation of U.S. President Bill Clinton says an ex-campaign aide to President Donald Trump should prepare to testify before a grand jury despite on-and-off threats about defying a subpoena.

Sam Nunberg said for a time Monday he would defy special counsel Robert Mueller, but by the end of the day said he would likely co-operate with a probe. In some interviews, he said there may be incriminating evidence against the president.

Susan McDougal served 18 months for civil contempt after prosecutors said she didn't testify honestly before a grand jury. She told The Associated Press on Tuesday that Nunberg's appearance before a grand jury is virtually assured because of his remarks.

A jury convicted McDougal, her husband and a sitting Arkansas governor during a 1996 trial.