If you give the available answers with your question you are more likely to get the right response
A census of the entire American population is held. This is basically a head count to see how many citizens the United States of America have. Based on the results, it is decided how many representatives will be in the house of representatives, so it changes often.
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The article in Politico suggests a couple of possible reasons for the celebrity involvement in the 2012 election like for one Barack Obama's star power and liberal appeal plus because of the social media boom which makes the communication much, much easier. Finally this kind of communication with the stars also works well.