“I’m not a clone of my father,” he said in a late-night interview after the forum earlier this month. His shirt was rumpled. He looked weary.

But with just weeks to go until his primary in the First District, Mr. Sanders, 49, still cannot avoid comparisons to his father, Bernie Sanders. His father, who won the 2016 presidential primary in this Live Free or Die state by 22 points. His father, whose booming, sometimes bellicose style on the stump seems to have rubbed off on his son.

Like Bernie Sanders, Levi Sanders is fighting — loudly — for single-payer health care, a $15 minimum wage and tuition-free public college. He vilifies corporate PAC money. He has asked backers to contribute $27, the same amount his father had touted during his presidential campaign to prove his support came from small donors. On Levi Sanders’s website? A photo of his father, front and center.

Mr. Sanders’s campaign, for a House seat in a competitive district, has puzzled everyone from the most enthusiastic Bernie supporters to his 10 Democratic opponents, more than a few of whom seem all too eager to dismiss his candidacy. Some people view him as a firebrand, with a quick fuse and a sharp tongue. Others describe him more as a nuisance, who has only gained notice because of his name. Because he lives outside the district, on the other side of this outsider-averse state, he has even been accused of carpetbagging.

Mr. Sanders, who is not a lawyer, has made a career in legal services fighting for the less fortunate. But there is no doubt he is a severe underdog in his primary. He worked for his father’s campaign, but in terms of non-familial political experience, he has none. Nearly a decade ago, he ran for City Council in Claremont, where he lives, and finished seventh in a field of nine.