Complex sentence: Rossiter became prominent as a moderate voice among the faculty, urging some understanding of the African-American students' frustrations but was branded a traitor by others on the faculty, some of whom (such as Allan Bloom) refused to speak to him again.
Split into simpler sentences: Rossiter became prominent as a moderate voice among the faculty, urging some understanding of the African-American students' frustrations. For this he was branded a traitor by others on the faculty, some of whom (such as Allan Bloom) refused to speak to him again.