Again, they have a completely different basis, so "extremely close to each other" is not so much anything but incorrect. But I know, nuance. I suppose my understanding of your interpretations is now much easier as well.
As for herd mentality, I'll try to be short, as this is no place for lectures on such a subject. Herd mentality is at its core defined as this;
Now, when it comes to ideological groups, say a political party or movement, herd mentality is directly tied to the ideological basis, and to inability to think out of its set boundaries, often resulting in a very antagonist stance towards anything that does not adhere to it strickly and firmly, be it a political party, a movement, an individual, etc. A glaring example would be RINOs, where often unaffiliated individuals are branded together for not adhering to the group's fundamental base, and where those who do are often not able to correctly and individually assess the situation thanks to their heir mentality, instead lashing out.The tendency for people's behaviour or beliefs to conform to those of the group to which they belong.