To Stephen Harper, he's Justin. To Tom Mulcair, he's "the third party leader."

In the House of Commons, MPs refer to each other by their riding names ("the member from Papineau"), and in public, they usually refer to each other with an honorific and their last names. But Harper, the Conservative leader, and Mulcair, the NDP leader, are more likely to refer to Liberal Leader Justin Trudeau as something other than "Mr. Trudeau."

Mulcair mostly avoids mentioning Trudeau's name at all, although when pressed he will refer to him as the Liberal leader — or, as he did Tuesday, the "third party leader," a reference to the Liberals' place in the House of Commons hierarchy. That is, beneath the New Democrats, who were the Official Opposition when Parliament was dissolved on Sunday.

Conservative spokesman Kory Teneycke gave a unique reason for the first-name treatment of Trudeau on Monday in an interview with The Canadian Press.

"The public views him as a celebrity and celebrities have one name: Oprah, Beyoncé, Bono, Justin," Teneycke said en route to Ajax, Ont.

On Tuesday, Harper pointed to the Liberals' own branding, which includes the website Justin.ca.

"I think I use that [Trudeau's first name] sometimes and sometimes I don't. That's how, in our experience, Canadians generally refer to him, because that's how the Liberal Party itself has branded him," Harper said.

Naming strategies

There are a few reasons why the other leaders may choose not to use the Trudeau surname:

Trudeau's father, Pierre Trudeau , was popular in some parts of the country, and the other leaders may not want to remind people of the link.

father, Pierre , was popular in some parts of the country, and the other leaders may not want to remind people of the link. Calling Trudeau , 43, by his first name emphasizes his relative youth and inexperience compared with Harper, who is 56, and Mulcair , who is 60.

, 43, by his first name emphasizes his relative youth and inexperience compared with Harper, who is 56, and , who is 60. Not using his name at all telegraphs a lack of importance in the overall campaign.

This campaign isn't the first time Mulcair has avoided using Trudeau's name. He was also asked about it nearly two years ago at an NDP caucus meeting in Saskatoon.

"You'd have to point to something that Justin Trudeau's ever done for me to be able to mention him," Mulcair told reporters.

One longtime Conservative supporter, speaking on the condition he not be named, said referring to Trudeau by his first name suggests a guy "that dates your daughter that you hate."

Trudeau called Harper's use of his first name a "distraction."

"That's what he's focusing on? I'm focusing on a real plan for growth, on investing in the middle class," he said of Harper.

"We have a time right now when the economy is struggling, when we're sliding into recession, when Mr. Harper has shown time and time again that his plan of helping the wealthiest Canadians is not working to grow this economy, and his distraction is working because I just got two questions on that rather than on how to build a stronger economy for the future of this country."

Avoid competitor's name

With Mulcair leading in most polls, the NDP leader's lack of references to Trudeau could be an indication their strategy is to target the incumbent rather than the party that trails them.

Debi Andrus, an assistant professor in the University of Calgary's faculty of marketing, said the NDP move can likely be explained because competitors don't like to mention each other to avoid confusing consumers — or in this case, voters.

"Every time you say your competitor's name, you're giving them more exposure," she said in an interview with CBC News.

But for the Conservatives, Andrus said, an emphasis on talking about Trudeau, however they name him, could raise questions in the minds of voters.

"Why would you do that and not talk about what you have to offer?... If you're talking about your competitor that often, what is it that you are hiding?"