Many political observers believe Americans are too cynical about politicians. Take it from someone who has been blogging for more than a decade and has met countless politicians and political aides: if anything, people aren't cynical enough.
1) The first priority of a politician is always getting re-elected: As Thomas Sowell has noted,
"No one will really understand politics until they understand that politicians are not trying to solve our problems. They are trying to solve their own problems — of which getting elected and re-elected are number one and number two. Whatever is number three is far behind."
Politicians may care about sticking to the Constitution, doing what's right for the country, and keeping their promises, but all of those issues pale in importance to staying employed in their cushy jobs.
2) Most politicians care far more about the opinions of interest groups than their constituents: Because of gerrymandering and America's partisan fault lines, even under the worst of circumstances, 75% of the politicians in Congress are in no danger of losing their seats to a candidate of the opposing party. Furthermore, because of their advantages in name recognition, fund raising, and the fealty of other local politicians to someone they view as a likely winner, most challengers from the same party have little hope of unseating an incumbent either.
The only way that changes is if an incumbent infuriates an interest group on his own side that has the money and influence to help a challenger mount a credible campaign against him. That's why politicians in non-competitive districts are far more afraid of groups like Freedomworks or the SEIU than their own constituents. Incumbents can -- and often do, crap all over their own constituents without fear of losing their jobs. However, if they infuriate an interest group, they may end up in the unemployment line.