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In his speeches as president , Obama did not make more overt references to race relations than his predecessors , but according to one study , he implemented stronger policy action on behalf of African-Americans than any president since the Nixon era .
 Obama, Nixon era
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Obama, Nixon era