OTTAWA (Reuters) - Canada’s governing Conservatives are likely to lengthen this year’s election campaign by launching it in August, three senior party sources said, a move that would benefit the cash-rich party.

Canada's Prime Minister Stephen Harper (R) and B.C. Premier Christy Clark visit a command post near the Westside Road wildfire in Kelowna, British Columbia July 23, 2015. REUTERS/Dan Riedlhuber

Canadians go to the polls on Oct. 19. Given that campaigns must last at least 37 days, the latest date Prime Minister Stephen Harper could start this year’s would be Sept. 13. Five of the last six campaigns have run about that length.

But the sources, who spoke on condition of anonymity, said Harper’s party already has its machinery in place and is expected to launch the campaign in August, possibly the first week. This would benefit the Conservatives, who last year changed a law that had imposed a maximum spending limit of around C$25 million ($19 million) on campaigns.

The new law increased the limit by about C$700,000 for every day beyond the minimum 37-day length.

The Conservatives have a larger donor base than their rivals and raised C$6.3 million in the first quarter, more than the two main opposition parties combined.

“A long campaign financially exhausts the other parties,” said one person familiar with Conservative strategy.

The right-of-center Conservatives, in power since 2006, are seeking a rare fourth consecutive victory. Opinion polls suggest they will find it tough to maintain their parliamentary majority.

Before the campaign starts, party spending is unlimited. The Conservatives have run many ads attacking their opponents. Despite this blitz, they have not been able to pull ahead in the polls.

The final decision on timing rests with Harper. Neither his office nor the Conservative Party responded to requests for comment.

An August call would prompt questions about why Harper wants such a long campaign, particularly one starting in midsummer when few are paying attention.

Senior Conservatives said one justification is that Canada, effectively, has already been in campaign mode for months.

Thomas Mulcair, leader of the opposition New Democratic Party (NDP), has been holding daily events since July 20, winning more media coverage than Harper. The NDP has a slight lead in many polls.

Asked on Tuesday what impact an early call would have on NDP funding, he said: “We will be ready for anything that happens.”

Infrastructure Minister Denis Lebel said on Monday that a longer campaign would allow the Conservatives to better lay out the contrasts between their policies and the opposition’s.

Critics charge that Harper - who came to power promising to increase accountability - is abusing government power for partisan purposes.

Ottawa increased child benefits last year and arranged for large checks to start arriving in July, accompanied by a flood of commercials paid for with taxpayer dollars.

All such government advertising must stop once an election is called, as would third-party advertising campaigns such as those being run by unions against Harper.

“If we go early, at least the opposition can’t complain that we’re campaigning with tax dollars as the government ... So there may be pretty compelling public rationale to go earlier,” one Conservative source said.

($1=$1.29 Canadian)