That't the kind of argument that are frequently encountered in websites and internet forums, but which receives little attention from more serious political philosophers. Basically, one person claims that the ideology some other person adheres to "really" is the same as some other ideology, that both persons despise. The "argument" laid forth for this claim is that both ideologies share some abstract similarity.
A social conservative might thus claim that "libertarians are really marxists at heart (they both wants to re-design society from abstract blueprints, showing little respect for established social mores, etc.); leftists can claim that "libertarians and fascists are really the same (since they don't share the class-war analysis, etc.)", etc. Proponents of virtually any ideology can say that virtually any two other ideologies "share the same fundamental ideas".
The problem is, of course, that though one can point to similarities between any two ideologies, one can also point at differences. To take a closer look at one of the alleged "similarities" between FDR liberalism and nazism:
- Second, that it's both possible and desirable to provide prosperity to favored groups by exploiting unfavored ones.
But nazis and FDR liberals disagree what counts as "favoured" and "disfavoured" groups, and what counts as "exploiting", as well as on many other issues. They are both similar (and so are also proponents most other political ideologies, like conservatives, social-demicrats, moderate libertarians, communists, etc.) that some state power is acceptable. But what state power, and what justification for this state power can be given, differ highly between the various ideologies. What one ideology counts as legitimate exercise of state power, may be regarded as illegitimate by another ideology. The similarities between the various ideologies thus boil down to that they don't think that a radical laissez-faire system (at least not always) is the most just political or economic system, and at least in some situations may render unwelcome results. But they may disagree highly on in what situations laissez-faire systems fail, which in turn follow from that the different ideologies may have very different ideas about justice, and what people are entitled to to begin with.