"I believe both your president and you, members of the board who supported this conduct, will be acting in the public's best interest by resigning your posts as soon as possible," Brown wrote. "While I suspect this request will fall upon deaf ears, I think you all owe the Purdue community and the general public a more thorough airing of the rationale for these recent moves than we have seen to date."
The vote followed revelations that Daniels tried to keep Howard Zinn's work out of Indiana K-12 classrooms while governor and wanted to ensure it didn't count toward teacher training credit.
A Purdue spokesman said the trustees had not yet read the letter and would not comment.
Daniels ordered staff to "disqualify the propaganda" used in teacher preparation courses in emails obtained by The Associated Press. He said he wanted to keep Zinn out of K-12 and fully supports academic freedom.
Daniels appointed most of Purdue's trustees. Keith Krach, who stepped down as board chairman minutes after approving Daniels' $58,000 bonus, said last week knowing of the emails during last year's presidential search would not have changed his mind on Daniels' selection.