Prime Minister Stephen Harper will call an election as early as this Sunday, kicking off what would be the longest federal election campaign in modern history, CBC News has confirmed.

The election is generally considered to be set for Oct. 19, 2015, under the Conservatives' fixed election law, although there is wiggle room. But Prime Minister Stephen Harper told Bloomberg News Wednesday that he considers that date to be set in stone.

Harper is expected to go to Rideau Hall Sunday or Monday to ask Gov. Gen. David Johnston to dissolve Parliament.

The Conservatives have sent out invitations to supporters for a rally in Montreal Sunday night. Harper is likely to head to Toronto after that.

Rumours about possible dates have swirled around Ottawa in recent weeks, partly because of a change in electoral law thought to benefit the Conservative Party.

Until last year, the spending limits were set no matter the length of a campaign, the minimum amount of time for which is 37 days (with voting day falling on the 37th day). But the Conservatives' Fair Elections Act provided for the parties' $25-million limits to be increased if the campaign is longer than 37 days: for each additional day the limit is increased by 1/37th, or an extra $675,000.

Candidates get an additional $2,700 a day to add to their usual limit of approximately $100,000.

'Don't speculate on my own actions'

Dropping the writ more than 11 weeks before voting day will make it the longest campaign in modern history. The previous longest campaign was a 74-day one in 1926.

Harper's interview with Bloomberg raised some eyebrows for the prime minister's assertion that he doesn't "speculate" on what he will do in the future. That came in response to Bloomberg's question about the election timing — a decision which rests entirely with the prime minister.

"I don't speculate, and I particularly don't speculate on my own actions.... Obviously, there is an important decision coming up for Canadians [on] Oct. 19," Harper said.

The first leaders' debate will fall in the first week of the campaign. The debate hosted by Maclean's magazine was already set for Thursday, Aug. 6 in Toronto, but hadn't been intended to fall during the campaign.

The campaign's timing could also mean Harper will be more available than usual to reporters and members of the public at the same time that his former chief of staff, Nigel Wright, takes the stand at the trial of Mike Duffy on Aug. 12. Duffy faces charges of fraud and breach of trust related to his time as a Conservative senator. He left the caucus to sit as an independent during the RCMP investigation into his expense claims.

Duffy and two other senators, Patrick Brazeau and Pamela Wallin, were suspended from the Senate during the investigation. The writ drop, however, dissolves the current session of Parliament and will result in the suspensions being lifted.

With files from The Canadian Press