To frazzle is an infinitive in your quotation. That's required after a linking verb. You can't substitute a conjugation for the infinitive.
The text means:
- The organization reacted to the impending California primary by becoming disorganized.
- This primary election will be the last of many Republican (or "GOP") primaries.
- This primary may decide who the party's nominee will be.
- This primary is a battle between Trump and his opponents, Ted Cruz and John Kasich.
The writer packed a lot into that sentence, as you can see by my explanation of it.
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