Complex sentence: Some of Griebel's opponents had focused on the fact that as a Republican candidate, Oz had made numerous donations to Democratic politicians, and as a result of this, his legitimacy as a Republican candidate for Governor had been under scrutiny.
Split into simpler sentences: Some of Griebel's opponents have focused on the fact that as a Republican candidate Oz had made numerous donations to Democratic politicians. As a result of this, his legitimacy as a Republican candidate for Governor has been under scrutiny.