Scott Brown

FILE - In this Nov. 13, 2012 file photo, then-Massachusetts Sen. Scott Brown speaks on Capitol Hill in Washington. (AP Photo/Alex Brandon, File)

CAMBRIDGE — The New Hampshire Senate campaign of Scott Brown is lashing out at a well-known Harvard Law School professor who said that, technically, Brown became a lobbyist for Wall Street last year.

Professor Lawrence Lessig is a political activist and founder of the Mayday political action committee, which sent out mailers to voters in New Hampshire pointing out that, when Brown lost his Senate re-election race to Elizabeth Warren in Massachusetts, he went to work for a law and lobbying firm called Nixon Peabody. The company said back in April that he was no longer working for them.

The Mayday PAC mailers called Brown a "Washington insider" and a "Former MA senator who sold his influence to a DC lobbying firm." It encouraged voters to pick Jim Rubens in the Republican primary to face incumbent Democrat Jeanne Shaheen.

In a strongly-worded letter dated Sunday, Sept. 7, and posted on Lessig's blog Monday morning, Brown's campaign calls the lobbying claim a "flat-out lie."

"Your willingness to trample the truth for your own partisan political agenda has added to the coarseness of our politics," the letter reads. "The people of New Hampshire can see through your cynical and thoroughly distasteful political attacks, but that doesn't excuse you from peddling total falsehoods."

It states that the campaign will explore "legal options" if the PAC does not stop sending the mailer, and questions Lessig's integrity in the context of Harvard's code of conduct. It's signed by Colin Reed, Brown's campaign manager.

In response, Lessig wrote that Brown's job may not have been a breach of Senate rules or even "lobbying" by the body's own definition, but for the average person, it's clear.

"I submit to anyone else in the world, a former Senator joining a 'law and lobbying firm' to help with Wall St's 'business and governmental affairs' is to make him a lobbyist," said Lessig. "Ketchup is not a vegetable, even if Congress says it is. What Brown did is lobbying, even if Congress says it isn't."

As far as the legal options threatened in the letter, Lessig says he's not scared.

"Let me offer the words of Harry Callahan," he said, in reference to the character popularized in film by Clint Eastwood. "Go ahead. Make my day."

He then posted a new image of a mailer that calls Brown a "Former Washington lobbyist and Massachusetts senator."

Scott Brown Campaign Letter to Lawrence Lessig re: lobbying claims