barack obama. >> the two go toe to toe in key primary states. >> nbc news has declared mitt romney to be the winner in wisconsin. >> in early april, santorum out of money and hope exits. >> and we will suspend our campaign effective today. >> by the end of april, the prize is now his. >> because after 43 primaries and caucuses, many long days and more than a few long nights, i can say with confidence and gratitude that you have given me a great honor and solemn responsibility. and together, we are going to win on november 6th. >> with the republican party's choice clear, president obama's campaign goes back to the future. stealing a page from bob schrum's 1994 strategy for senator kennedy seeking to define romney before romney can define himself. the obama team goes after romney again on bain and calling on him to do what his own father did, release multiple years of his personal tax returns. >> i think bain is the beginning of a narrative arc. bain will be around i suspect