Conservative Leader Andrew Scheer today called on Liberal Leader Justin Trudeau to resign as prime minister if his party does not win the most seats on election day, saying that practice has become a "modern convention in Canadian politics."

Scheer said that if Trudeau's Liberals slip to second place in the seat count after Monday's vote, he should step aside rather than try to pursue an arrangement with the NDP to hang on to power.

"It is quite clear that Justin Trudeau will try to do anything to stay in power," Scheer said at a campaign stop in Brampton, Ont.

"But what I'm saying is that the party that wins the most seats should be able to form the government and the other convention in modern Canadian politics is that a prime minister who enters into an election and comes out of that election with fewer seats than another party, resigns. That is a modern convention in Canadian politics."

Conservative Leader Andrew Scheer spoke with the CBC's Katie Simpson in Brampton on Thursday 1:08

Former prime minister Paul Martin resigned after the 2006 election handed Opposition leader Stephen Harper and his Conservatives a plurality of seats — but not a majority — in the House of Commons.

Pierre Elliott Trudeau also promptly resigned as prime minister after Joe Clark's Progressive Conservative party won a plurality of seats in 1979 election. Trudeau then rescinded his resignation as Liberal Party leader after Clark's government fell on a confidence vote about nine months later.

When asked in 2015 if the party with the most number of seats should have the right to try and govern, Justin Trudeau said yes.

"That's the way it's always been," Trudeau said in an interview with CBC News. "Whoever commands the most seats gets the first shot at governing. Whoever gets the most seats gets the first shot at trying to command the confidence of the House."

However, confusingly, in the same interview, Trudeau then said "absolutely" the "outgoing prime minister" should have the first crack at testing the confidence of the Commons.

Liberal Leader Justin Trudeau spoke to the CBC's Tom Parry in Trois Rivieres on Thursday 1:10

The talk of resignation comes as the Liberal and Conservative parties are locked in a battle for front-runner status. The CBC's Poll Tracker is projecting that neither party will secure a majority government on Oct. 21.

NDP Leader Jagmeet Singh has said he'd consider working with a Liberal minority government. Trudeau has only said he wants to secure the most seats.

Green Party Leader Elizabeth May said Thursday it is "astonishing" that party leaders "don't seem to understand our system."

She said Scheer is knowingly "misleading" Canadians about the country's parliamentary traditions.

"The convention is quite the opposite of what Mr. Scheer is telling people. I'm not advocating it. I'm explaining what the rules are. The convention is the party that held power before the election has first crack at seeing if they can hold the confidence of the House ... Mr. Trudeau, gets first crack at it," she said. "We elect 338 MPs and they have a right to decide who should form government at the end of an election."

In Canada's system of Westminster parliamentary democracy, the prime minister and the cabinet must answer to the House of Commons and they must enjoy the support and the confidence of a majority of the members of the chamber to remain in office.

Need support of the House

The sitting prime minister is given the first chance to test the confidence of the Commons after an election — even if that PM's party does not command a majority of seats.

For example, recent elections in B.C. and New Brunswick produced very close results, with the two main parties all but tied for first place in the seat count. The sitting premiers tested the confidence of the provincial chambers to see if they could secure enough votes to pass a throne speech.

In both cases, the government failed to win the required support from provincial legislators and was defeated by a vote of non-confidence.

According to convention, in such a scenario the premier must either resign or call for the dissolution of the chamber to allow for a new election. The same is true for a prime minister at the federal level.

NDP Leader Jagmeet Singh spoke in Welland Ontario on Thursday 0:47

However, in both B.C. and New Brunswick, the non-confidence vote came quite soon after an election. So the lieutenant-governors in those provinces turned instead to other party leaders to ask them to assemble enough votes to pass a throne speech.

In B.C., the Green Party agreed to support the governing NDP. In New Brunswick, the Progressive Conservative party has relied on support from the People's Alliance.

Asked by reporters Sunday whether he'd work with other parties, including Trudeau's Liberals, Singh replied, "Oh absolutely, because we're not going to support a Conservative government."

"We're going to fight a Conservative government, gonna fight it all the way," the NDP leader said at a rally with supporters in Surrey, B.C. "So we're ready to do whatever it takes."

Trudeau has been asked repeatedly about Singh's comments but, so far, he hasn't publicly said he supports the idea of a coalition government or of governing as a minority with New Democrat support.

Green Party leader Elizabeth May says Conservative Leader Andrew Scheer is being misleading for suggesting that if his party gets the most seats, he will get to form government. May answered reporters questions while campaigning in B.C. Thursday. 1:12

"Our focus is on electing a progressive government, not a progressive opposition, and ensuring that we stop Conservative cuts," Trudeau said.