What forces fight against the progress of democracy?

The primary force fighting against democracy is human nature. Whether or not you consider human nature a 'political force' is an open question, but it's the place to start. The progress of democracy is sporadic because learning to harness human nature in a productive fashion is difficult.

Democracy will be more successful when we devise a method of selecting political leaders that makes integrity an important character trait - to those who seek public office and to those who select them. The candidate selection process must ensure that office-seekers are carefully examined by peers seeking the same office. When that happens, the candidates will have to exhibit not only their ability but their probity, if they want their peers to elect them.

In such an environment, it is in the candidates' best interest to maintain their own integrity. That is the way we can harness human nature to improve democracy and benefit society.