detect allusions (e.g. very fuzzy matches) in language of inaugural addresses

If you are inspired to use bigrams, you could build your bigrams while allowing gaps of one, two, or even three words so as to loosen up the definition of bigram a little bit. This could work since allowing n gaps means not even n times as many "bigrams", and your corpus is pretty small. With this, for example, a "bigram" from your first paragraph could be (similar, inaugurals).