Liberal MP Marc Garneau has accused Stephen Harper of making it legal to overspend on the upcoming federal election campaign, as rumours swirl that an election call will come this weekend.

Garneau slammed the Conservatives for raising campaign spending limits for all candidates, thereby making it easier for the Conservatives to spend their sizable war chest of campaign money. Garneau also pointed out that more than half of what gets spent in the election will be recouped from the taxpayer through Elections Canada rebates. That means the longer the election runs, the more money all parties can spend and recoup through rebates.

"Stephen Harper knows that an early election call benefits him and the Conservative Party, while Canadians pay the price," Garneau said at a news conference on Friday. "The Harper Conservatives continue to spend taxpayers' money for their own political gain."

The PM could trigger the election as early as Sunday, right in the middle of the August long weekend. The minimum length of an election campaign is 37 days, but if Harper sticks to the scheduled Oct. 19 election date, then the election campaign would be 11 weeks long.

Garneau says such a lengthy campaign would primarily benefit the Conservatives, who stand to rake in "the biggest taxpayer-funded rebate in Canadian history, up to $25 million."

Here's a look at what a long election campaign could mean for Canadian voters.

More time to spend

According to the latest poll from Nanos Research, the Harper Conservatives are neck-and-neck with the New Democrats in several key areas. However, an early election call could benefit them in the long run, pollster Nik Nanos says.

Nanos points out that the Conservatives have raised more money for the election than any of their rivals, and a prolonged election campaign would allow them to outspend their opponents and hammer home their election messages through advertising. It would also force the other parties to make every cent count.

"They obviously believe that an early election will benefit them," Nanos told CTV's Canada AM on Friday. "They know they can outspend the other campaigns."

The Conservatives have already positioned themselves to outspend their opponents in this election. They passed legislation during their latest term that will allow each party and candidate to spend more money after the first 37 days of the election, up to $675,000 per day.

That will allow the Conservatives to flex their financial muscle in the second half of the campaign, when the other parties may start to feel the financial crunch.

Nanos says time and money will be major factors in the election. "Watch to see when the parties decide to really ramp up their spending," he said.

The last two election campaigns have stuck to the 37-day minimum length, but the 2006 election campaign stretched over Christmastime and lasted 55 days.

Nanos says the Conservatives "front-loaded" their spending that year, "and by Christmastime, the tank was almost empty because they had spent a lot of money on advertising, and hadn't moved the numbers."

Like a runner in a marathon, each party will have to choose when to sprint and when to jog.

Taxpayers foot the bill for a longer campaign

Taxpayers will be on the hook for 60 per cent of all the money spent during the election, thanks to an Elections Canada rebate offered to all parties and candidates. That means a longer campaign with looser spending rules will allow each party to dip deeper into the federal coffers.

Jean-Pierre Kingsley, Canada's former chief electoral officer, said he "sincerely hopes" Harper does not stretch the election campaign beyond 37 days.

"That will mean up to $50 million or more dollars that political parties can spend," he told CTV News Channel. He added that a longer campaign would be a "significant advantage for the Conservative Party of Canada," but it would also reflect poorly on the PM.

"They're doubling their opportunity here to dip into the taxpayer's pocket," Kingsley said.

In addition to the rebates, Elections Canada will also be forced to spend more money in administrative costs over the course of an 11-week campaign.

Fewer third-party attack ads, more party-driven ones

An early election call would allow the Conservatives to put a muzzle on some of their harshest third-party critics, including a number of unions and issue-driven lobby groups. Those third parties can spend as much as they want on attack ads before the election campaign, but once an election is called, their spending comes under tight restrictions.

That means most of the attack ads run during the election will come from rival parties, instead of from those anti-Conservative interest groups.

At Garneau's news conference, the Liberal MP accused Harper of wanting to "reduce his critics to silence" by dropping the election writ.

Nanos also said the Conservatives are eager to silence their third-party critics.

But while those third-party ads are expected to all but disappear, Canadians can expect to see many more party-sponsored attack ads taking their place, especially once the purse strings are loosened up in the latter half of the campaign.

"We're going to be seeing a lot more of those," Kingsley said.

Economic woes

If Harper triggers an election this Sunday, it will come in the middle of a downturn in the Canadian economy that could deepen into a recession over the course of the campaign.

The latest numbers from Statistics Canada show the economy shrank for a fifth straight month in May , increasing the possibility that Canada fell into a recession in the first half of 2015. Those numbers come despite the Harper government's promise of a balanced budget in 2015.

June's GDP numbers will come out at the end of August, and if the numbers are down, Canada will officially have fallen into a recession at the tail end of Harper's administration.

But whether or not the recession becomes official, the economy will remain a key point of contention.

According to the latest Nanos Research poll for CTV News, respondents considered Harper and NDP Leader Thomas Mulcair the two most trusted leaders to manage Canada's economy. Harper and Mulcair each received 31 per cent of the support in a poll asking who respondents trusted most to manage the economy. Liberal Leader Justin Trudeau trailed with 18 per cent support.

Nanos says those numbers could mean trouble for Harper, who once enjoyed a lead in that poll. "I would say for the Conservatives, it's actually quite risky at this particular point in time," Nanos said.

No guarantees

Nanos says the Conservatives' considerable war chest may not be enough to get them elected again. "When you go into an election campaign, anything can happen," he pointed out.

He added that the current polls are "not favouring the Liberals," but the party will have plenty of time to make up ground over its rivals during the campaign. "Right now, the Trudeau Liberals are at a 12-month low," he said.

On the other hand, the NDP are on the rise, bolstered by a recent surge in party fundraising and popularity.

Nanos says despite all the posturing right now, most parties will wait until the latter part of the campaign to really ramp up their efforts.

"Even with an early election call, the real campaign will only intensify probably after Labour Day," he said.