He is a luminary in the world of cyberlaw, a star Harvard professor with a résumé a hundred pages thick, and a sensation on the thought leader circuit. But even though he has raised more than $1 million for his presidential bid, Lawrence Lessig, who is mounting a quixotic campaign for the Democratic nomination, is struggling to get noticed.

He was excluded from his party’s first debate on the grounds of weak poll numbers, while many surveys have not bothered to ask voters about him. As two rivals, former Senator Jim Webb of Virginia and former Gov. Lincoln Chafee of Rhode Island, dropped out of the race last week, narrowing the Democratic field, Mr. Lessig still could not cadge an invitation to the annual Jefferson-Jackson Dinner in Iowa on Saturday night. And while they have no doubts about his intellect, even some of his academic friends question his intentions and whether he is really of presidential timber.

Still, that does not deter Mr. Lessig, 54, whose campaign has a singular focus on an issue that polls show deeply resonates with voters: overhauling the campaign finance system, which he calls the root of America’s problems.

“It’s been my whole life to focus on this issue and get people to recognize this fundamental point first,” Mr. Lessig said, referring to the idea that nothing in politics can be accomplished until a corrupt system is cured. “One of the hardest challenges here is to have the chance to make the point I want to make.”