New York State voters rarely have enough real choice on their ballots, and Gov. Andrew Cuomo seems determined to keep it that way. Ever since another Democratic candidate for governor, Zephyr Teachout, arose on his left, Mr. Cuomo has tried to throw her off the primary ballot on the shaky grounds that she hasn’t met the state’s five-year residency requirement.

On Monday, a Brooklyn judge tossed that argument out of court, ruling that Ms. Teachout is perfectly qualified to be on the Democratic primary ballot on Sept. 9. But Mr. Cuomo, true to form, won’t back down. His aides immediately promised an appeal of the ruling. That is political bullying, and the governor should back off and engage with Ms. Teachout as a serious candidate. Doing otherwise suggests he is more nervous about winning a second term than he would like to appear.

Ms. Teachout, who grew up in Vermont, has energized her audiences with humor and biting criticism of the governor’s ethical failings. Once a campaign aide for Howard Dean, she says she supported Mr. Cuomo in 2010 but now condemns him for meddling with the Moreland Commission he appointed to investigate Albany corruption. She also criticizes him for his “tax breaks for the rich,” his failure to reform campaign finance and redistricting, and his failure to pass laws protecting women’s equality. If elected, she promises to bring back the “trustbusting” era of the Roosevelts.

Although Mr. Cuomo is leagues ahead in the polls and has a $32.5 million bankroll, he should not dismiss Ms. Teachout and her growing number of followers as irritants. Her criticisms are mostly legitimate, and he should defend his first term in a series of robust debates with her in the weeks before the primary, rather than through the timidity of litigation.