Ivor Williams stood on his porch and stared suspiciously at the visitor who was pointing to the attic of his pink and shingled house near Brooklyn College.

“I used to live here,” Steve Slavin explained, to no noticeable reaction from Mr. Williams. Then he uttered the magic words: “With Bernie Sanders.”

“Bernie?” Mr. Williams, a 78-year-old immigrant from Guyana, exclaimed. “I was just watching Bernie on the TV.”

Hillary Rodham Clinton may be a former senator from New York who located her campaign headquarters in Brooklyn Heights, but all it takes to know who really represents Brooklyn in the race for the Democratic nomination is for Mr. Sanders to open his mouth and utter a few syllables.