“Strange Political Amalgam in Ambitious Young Man,” read a 1972 headline about him in The Akron Beacon Journal.

“He defends Trump, sees UFOs,” read another from The Cincinnati Enquirer in March, alluding to Mr. Kucinich’s recent career as a Fox News pundit and his claim to have seen an unidentified flying object at the home of a friend, the actress Shirley MacLaine. “Can Kucinich win?”

He can, however improbably. Mr. Kucinich has emerged as the most persistent threat to Mr. Cordray, the presumed favorite, spooking party officials who fear Mr. Kucinich would stand little chance in the fall. Primary polling has been inconsistent: One recent survey showed the race effectively tied; another gave Mr. Cordray a double-digit edge, though more than half of respondents were undecided.

Mr. Kucinich is betting on a coalition of still-loyal former constituents: A brief stroll through Cleveland’s West Side Market recently included interactions with a woman whose father Mr. Kucinich once caddied for, a produce vendor who attended school with his sister and a police officer who worked security at his third wedding. He has also built a nascent following awakened to his politics since Mr. Sanders’s run.

Mr. Sanders himself has not endorsed Mr. Kucinich. “Dennis is his own man,” Mr. Sanders said in an interview with journalists from The New York Times, calling him an old friend. “Dennis is a very — what’s the word? — unusual politician.”

But Our Revolution, the Sanders-aligned group, has held up Mr. Kucinich as a kind of progressive seer, who pushed ideas like tuition-free college long before they came into wider fashion, rallying behind the storm-the-castle populism familiar to admirers of both men.

In an interview, Mr. Kucinich rejected any suggestion that this is a race between two generally analogous progressives on policy. He described himself, unsubtly, as a leader “for a couple of decades” on issues like single-payer health care. And since the Parkland, Fla., massacre in February, he has made gun safety central to his campaign, repeatedly reminding voters of Mr. Cordray’s past “A” rating from the National Rifle Association and his refusal to embrace an assault weapons ban.