What inhibits a winning candidate from doing exactly the opposite of what he promised?

Answer 1

Nothing. According to most political theory, we can expect that Candidates will act in a manner that will keep them elected. As a result, if an elected official either has no need for reelection or will not be punished for their deviation, then one can assume the official will act according to their own desire. That being said, elected officials tend to want to at the very least create the appearance of adherence, so it is unlikely an elected official will outright ignore the will of their constituents.