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His prominent role in campaigning for President Obama during the 2012 presidential election and his widely publicized speech at the 2012 Democratic National Convention , where he officially nominated Obama and criticized Republican nominee Mitt Romney and Republican policies in detail , earned him the nickname " Explainer-in-Chief " .
 Obama, 2012 presidential election, Obama, Mitt Romney
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, 2012 presidential election, Obama, Mitt Romney