The answer to this question seems to depend on how long the politician has been in office.
When they are first elected, most politicians seem to have a genuine interest in representing the causes of their constituency. Foremost on their mind is to get things done.
When they come to the end of the first term and decided they want to continue, foremost on their minds seems to be re-election. If they are a Congressman or woman this pattern repeats itself every two years ad nauseum. Or, their scope of vision widens and they seek to be elected to a higher office. Still, their minds are mostly focused on being re-elected or elected to a higher office.
Along with the above, these same politicians think extensively about how to keep their donors happy. So, they lose sight of their constituency in order to retain the favor of their sugar daddies. If they are on the Right they look to folks like the Koch Brothers or Karl Rove. On the left they want to please George Soros and folks in Hollywood.
Perhaps as they near the end of their legislative lives (as many incumbents seem to be re-elected for life whether they have gotten anything done or not), they may actually begin thinking about their legacy and attempt to get something done.