Washington (CNN) Sen. Bernie Sanders is a political independent, who proudly calls himself a socialist. As he declared his presidential candidacy Thursday, he pledged to run on the Democratic ticket.

He could hit an early roadblock in New Hampshire -- not with Hillary Clinton, but William Gardner, who has guarded the state's first-in-the-nation presidential primary for four decades as Secretary of State. He said he isn't sure whether Sanders meets the state's requirement to be on the presidential ballot.

"If they're going to run in the primary, they have to be a registered member of the party," Gardner told CNN. "Our declaration of candidacy form that they have to fill out says 'I am a registered member of the party.'"

Gardner, who takes pride in personally greeting all presidential candidates in the fall when they file their paperwork at the State Capitol in downtown Concord, N.H., stopped short of saying Sanders would be excluded from the 2016 Democratic primary ballot. But he said he did not know how Sanders could answer the simple question on the form: Are you a registered Republican or Democrat?

"We have only two legal parties in New Hampshire," Gardner said in an interview. "The primary is only for those legal parties."

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