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The Democratic nominating contest is getting lonely for Lawrence Lessig.

Mr. Lessig, the Harvard law professor who rails against unfairness in campaign finance, shifted his message to the unfairness in the debate selection process and the fact that he had not been invited to next week’s gathering in Las Vegas.

“I’m surprised by the lack of recognition from the Democratic Party,” Mr. Lessig said in an interview. “It’s unclear how if you’re not a politician or a billionaire you get to a place where you are able to participate.”

Mr. Lessig said that his campaign had more “energy” than those of Lincoln Chafee, Jim Webb or Martin O’Malley — who are expected to face off against Hillary Rodham Clinton and Senator Bernie Sanders of Vermont — and that the fact that he was able to raise $1 million in a month shows that he is a legitimate candidate.

He said his lack of an invitation to the debate was a “Catch-22” because the criteria were based on polls that did not include him. Failing to make the debate stage based on those rules makes it harder to gain notoriety, potentially putting his bid out of business before it had a chance to get going.

Mr. Lessig is spending money, and he kicked off his advertising campaign this week with a broadside against Senator Marco Rubio of Florida. He plans to go after Jeb Bush and said that Mr. Sanders and his other rivals were all talk when it came to campaign finance.

In the meantime, Mr. Lessig said he and his family had been scraping by since kicking off his campaign last month. Harvard cannot pay his salary while he is on leave to seek office, and campaign finance rules say that he cannot pay himself with donations until November.

“I can’t even borrow money, except from credit-card companies,” said Mr. Lessig, who is married and has three young children. “The assumption was that I would be making the money in the family.”

Hopeful that he will still be asked to join the debate, though he has not been in contact with its officials, Mr. Lessig said he had been brushing up on a range of issues with his campaign consultants and that he was prepared. And he says that he is ready to talk about more than just campaign finance.

“I’m not going to be the ‘rent is too damn high’ candidate,” Mr. Lessig said, referring to his crusade to overhaul campaign finance. “But the objective would be to make it central and make the priority of resolving it inescapable.”

If he is shut out of the first two debates, however, Mr. Lessig acknowledged that it would most likely be time to pack it in and write about the experience.

“I didn’t intend this as a research project, but I’m sure after it’s over I’ll be reflecting on the craziness of it,” he said.