The Cuomo campaign had sought to disqualify Zephyr Teachout, a Fordham University law professor and liberal activist, on the grounds that she has not lived in the state for the past five years, as required by the state constitution.
Justice Edgar Walker ruled that Teachout has been living in New York since June 2009 despite maintaining a Vermont driver's license.
The Cuomo campaign plans to appeal.
The primary is scheduled for September 9.
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