"I consider my voice to be a centrist moderate voice among the nine Democratic candidates"

Bob Graham's quote recommends that he thinks his political views are more moderate than those of the other Democratic prospects. He is most likely referring to the reality that he is not as far left on the political spectrum as a few of the other candidates. He is most likely trying to attract more moderate voters who may not agree with the more progressive policies of some of the other prospects. By placing himself as a centrist, he is wishing to attract more moderate voters who might be switched off by the more severe views of some of the other candidates. By doing so, he is wanting to get more assistance and ultimately win the nomination.