Frank Hall, whom Del Mastro contracted to do voter outreach and identification during the 2008 federal election campaign, said Del Mastro used his parliamentary privilege in the House of Commons last week to attack Hall's credibility.
"In his point of privilege, Mr. Del Mastro gratuitously slandered me and my character on the floor of the House of Commons, where he enjoys immunity as a member of Parliament," Hall wrote in the letter to Scheer, also provided to CBC News.
"Mr. Del Mastro enjoys privileges not afforded to a private citizen — the ability to speak about other Canadians in the House of Commons without being held to account for the inaccuracy of his remarks. He is aware that I have no legal right, or ability, to defend myself from his slander."
Hall also sent a letter to the prime minister.
Del Mastro was complaining last week in the House about an investigation by Elections Canada into his 2008 campaign spending and arguing his privilege has been breached by leaks to the media.
Elections Canada is investigating Del Mastro's spending in the 2008 campaign, according to court documents released last year.
Asked what he thought of Hall's demand that he apologize, Del Mastro said MPs must be allowed to "re-establish" their reputations when they have been tarnished.
"Parliamentary precedence in this matter is clear and absolute," he wrote in an email to CBC News.
Hall said Del Mastro is trying to interfere with the investigation by Elections Canada, for which he is subject to a court order that required him to hand over information to the agency.
Hall said Del Mastro's statements about him in the House of Commons were "character assassination" meant to dissuade him from participating with Elections Canada in the investigation and could intimidate other witnesses too.
"As Speaker of the House of Commons, you should be seriously concerned about the effects of these remarks on those proceedings, and do something to protect me and the other Canadians who have been ordered to co-operate with this and other investigations," Hall wrote to Scheer.
Hall said he wants Scheer to order Del Mastro to withdraw his comments and apologize in the House.
In the letter to Harper, Hall said Del Mastro was trying to use his parliamentary privilege "to bully, intimidate, malign and discredit a witness."
"In closing, Mr. Prime Minister, you, Mrs. Harper and your government have embraced the very worthy anti-bullying movement. I would ask that you ensure that the conduct of your parliamentary secretary reflects these noble efforts," Hall wrote.
Questions about campaign spending
Court records show Elections Canada is looking into a personal cheque for $21,000 Del Mastro wrote to Hall's now defunct company, Holinshed.
Del Mastro's campaign also wrote two cheques to Holinshed for a total of $21,000. The records show one was returned and one cancelled.
Candidates have a limit of $2,100 they can spend on their own campaigns. Del Mastro said the campaign reimbursed him, but his campaign spending file, which includes receipts, cheque stubs and banking records, at Elections Canada shows no sign of that repayment.
The file also indicates that if the campaign reimbursed him for that $21,000 personal cheque, it would be over its spending limit for that election.
Del Mastro has told CBC News that all of his campaign's spending was above board and that he has done nothing wrong. He said some of the spending was for his riding office in Peterborough, Ont.