i'm jim sciutto in today for fredricka whitfield. we are following new details on a major political story today. donald trump breaking his silence with a series of tweets for the first time since he failed to correct a supporter who called president obama a muslim, seemed to insult other muslims in america. trump tweeting this morning, "am i morally obligated to defend the president every time somebody says something bad or controversial about him? i don't think so!" he is scheduled to speak tonight in iowa. cnn's sunlen serfaty is at the michigan republican leadership conference on mackinaw island. sunlen, trump sent out a series of tweets. it's what he said, but also what he didn't say, isn't it? >> reporter: absolutely, jim. that's absolutely right. it's what he didn't say that really is the important part, you know, i should note first, that his defense today where he's pushing back for the first time since that moment on thursday in new hampshire, it's a very different pushback than what his campaign originally said. his campaign originally saying he didn't hear the question, and that's why he didn't necessarily correct it in the moment.