Is it wrong for politicians to "do anything" to get re-elected?

This is a difficult question to frame. We all hate when a politician says he’ll do one thing, then votes differently when the time comes. However, if popular opinion on an issue in his/her district changes, should that politician’s vote just echo the sentiments of the majority of the constituents, or remain “true” to party lines? Put another way, should the politician really just be there to vote the current will of the people, or should they remain faithful to the doctrines of their political party? It occurs to me there is a certain logic to politicians holding no opinion and just following the will of the people, but it is also logical for people to vote with a party whose ideals they expect to see reflected by the elected official’s subsequent voting record. Which system do you think is best?

If possible, keep this discussion theoretical and don’t start attacking Obama or the Tea Party.