If there is one thing rightwing political parties consistently do wrong is not taking enough distance from fascism. Recent developments in the UK and the Netherlands show this once again.
If there is one thing leftwing politicicans consistently do wrong is not taking enough distance from communism. The main moticiation for leftwing social-democratic politics should be the protection of diversity in a society. Where rightwing politicians think that economical freedom automatically leads to diversity, leftwing politics is aware that for the protection of diversity, one should guarantee affordable access for everyone to main aspects of a society (e.g. health, education, justice). Communism, on the other hand, never has protected diversity, quite the contrary. This is both true for communist countries (there is not a single example of a communist country that didn’t actively suppress diversity) as for communist parties in democratic countries (e.g., the former CPN in the Netherlands, CUP in Spain) which always part from their main dogma that their party represents “the people” and hence not agreeing with them automatically will make you an enemy of “the people”.
Any political movement that speaks in slogans of “the people” goes fundamentally against the basic idea of democracy: diversity. It is therefore unfortunate that nowadays almost all political parties, never mind their orientation, claims to represent the “will of the people”.