“They would say that about anyone who was contemplating doing this as a relative unknown,” Mr. O’Malley said. “But history is full of relative unknowns who go and do the hard work, put together a more compelling framework for our country’s future, and go out and campaign.”

The campaigning part, at least, Mr. O’Malley has down. The governor gets the midterms attendance ribbon, having campaigned for more than 150 Democratic candidates and visited Iowa four times since June. His political action committee has paid for staff on the ground, including in Manning, S.C., where on a recent afternoon he campaigned for the Democratic candidate for governor, Vincent Sheheen. As Mr. O’Malley slipped into a room in the back of the Trinity A.M.E. Church, crammed with about 100 mostly African-American voters, his communications director, Lis Smith, located a local reporter in the room.

“Let me know if you want to chat,” she told him.

Mr. O’Malley stood behind a small lectern at the center of the room and declared, “Greetings from Maryland,” which was met with near silence, and “in particular Baltimore,” which was met with applause. He knew his audience. “That city that the young Frederick Douglass loved, that city that Thurgood Marshall loved,” he continued.

Mr. O’Malley has an unorthodox style on the stump: He often pans the crowd with a fixed and blank gaze. His dramatic language and gestures can seem inauthentic, almost hammy. (This was most widely noted in his 2012 Democratic National Convention speech, in which he used kindergarten teacher intonations to lead the crowd in repeated calls and responses of “moving American forward — not back.”) On a more personal level, he can be engaging when he relaxes, but at times he maintains unblinking eye contact while speaking in whispered sentences with telepathic intensity.