Washington (CNN) Bernie Sanders isn't running to push Hillary Clinton to the left, the senator said on Thursday, rejecting the idea that his long shot candidacy is aimed at issues, not winning.

"This is not an education campaign. This is not a protest campaign," an animated Sanders said at the Christian Science Monitor breakfast in Washington. "This is a campaign to win."

Since announcing his campaign for president in late April, Sanders has been greeted by sizable crowds in early voting states like Iowa and New Hampshire. Though he finds himself polling far behind Clinton, Sanders has been her most potent progressive challenger so far.

But political watchers and pundits have suggested that Sanders's primary effect on the race will be to push the former secretary of state to the left on issues, not mount a credible challenge to her for the Democratic nomination.