syntax - Which phrase is grammatically correct?

Only one of the following is correct, but I am not sure which. My answer key says that Sentence 1 is correct, but I don't understand why. Here they are:

Sentence 1: Therefore, a Platonic idealist believes that these abstract forms are as effective as, if not more effective than,
sensory experience at revealing the nature of reality.

Sentence 2: Therefore, a Platonic idealist believes that these abstract forms are as effective, if not more effective, than
sensory experience at revealing the nature of reality.

I still think sentence 2 sounds more syntactically accurate.

Answer

You're injecting a clause into the middle of an already complete sentence. This means, if you remove the clause, does the sentence still make sense?

• ... forms are as effective as sensory experience ...
  
  


• ... forms are as effective than sensory experience ...