I would liek to add something to this that would clarify things a bit. "the more so" in this context means "all the more so". The "all" in the phrase can be dropped.
The sentence can then be
Like the Dreyfus affair at the turn of the century and the Sacco-Vanzetti trial in 1921, the Hiss case became the political cause of an era, all the more so because it was the first time a congressional hearing was shown on television.
Which sounds a lot better and makes more
...
The sentence can then be
Like the Dreyfus affair at the turn of the century and the Sacco-Vanzetti trial in 1921, the Hiss case became the political cause of an era, all the more so because it was the first time a congressional hearing was shown on television.
Which sounds a lot better and makes more
...
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