By Paul Vieira 

OTTAWA--Canadian Prime Minister Stephen Harper on Sunday launched an election campaign that will see the Conservative leader seek a fourth consecutive mandate on Oct. 19 amid a backdrop of deteriorating economic conditions and voter fatigue.

Mr. Harper is the second-longest serving leader among Group of Seven country leaders. His bid for re-election comes as Canada's economy has stalled amid steep declines in the price of oil, its top export, and as a serious election threat emerges from the left-leaning New Democratic Party, which currently leads in the polls.

Mr. Harper has tied his electoral success since coming to power in January 2006 to a narrative that his party is best suited to oversee economic policy. Canada emerged relatively unscathed from the 2008-2009 global financial crisis and voters rewarded Mr. Harper with a majority mandate in parliament in the last national election in May 2011, when his party attracted 40% of the popular vote.

The Canadian leader faces more difficult economic conditions this time around. The Canadian economy posted negative growth in each of the first five months of 2015 as lower prices for oil and other commodities sideswiped trade and business investment, pushing the Canadian dollar sharply lower and, according to some economists, putting the country on track for a recession in the first half of the year.

Mr. Harper's pro-trade agenda also suffered a blow in recent days when members of the Trans-Pacific Partnership failed to clinch a deal in Hawaii. Mr. Harper had said a TPP deal was crucial for Canada's economy.

Mr. Harper, 56 years old, said Sunday that management of the economy was his party's top priority, and stressed his experience in dealing with the economy and national security as a crucial advantage over his rivals. He said a lengthy campaign was needed so Canadians could explore the options and make a "serious choice."

"This is an election about leadership on the big issues that affect us all," Mr. Harper said. "It is an election about who will protect our economy in a period of ongoing global instability and secure Canada's prosperity. And it's about who is best equipped to make the tough calls to keep our country safe."

Canada's election date was fixed for Oct. 19, but the prime minister has the power to decide when to dissolve parliament and start the official campaign. An 11-week campaign--the longest in Canadian history and double the length of the recent U.K. election campaign--plays to the Conservative Party's strengths because it has a bigger war chest than its two main political rivals. Since 2012, Mr. Harper's Conservatives have raised 69 million Canadian dollars (US$52.8 million), or nearly as much as their two main rivals combined, according to data from Elections Canada.

The Conservatives' financial edge may help overcome some voter fatigue with the party, which has been in power for more than nine years.

The opposition NDP enjoys 34% support among Canadians, a modest lead over the Conservatives' 30%, according to a recent poll from Ottawa-based Ekos Research. That firm and others have also released survey results indicating up to two-thirds of Canadians believe it is time of a change in government in Ottawa.

The left-leaning NDP has never been in power at the federal level, although it has governed at the provincial level, most notably winning the recent provincial election in resource-rich Alberta. The national party is led by Thomas Mulcair, a lawyer and former environment minister in the province of Quebec.

Mr. Mulcair said Sunday that his campaign would be focused on providing support to Canadians struggling under stagnant wage gains and increasing levels of debt, through policies such as affordable day care.

Mr. Mulcair also said he would take measures to support Canadian manufacturers, which are struggling to sell goods abroad even amid a weaker currency and a pickup in U.S. demand, and attract new factory investments into Canada.

The Liberal Party of Canada is in third place in the polls, in the mid-20% range. In a statement, its leader, Justin Trudeau, said it was time for a new government with a "plan to grow the middle class and grow the economy."

Analysts say for Mr. Harper to win, he must count on the coalition of voters that brought him a majority in 2011--and that includes western Canada, suburban voters in the Toronto region, and immigrants.

Still, Mr. Harper "is in the hunt, and doing a lot better than you would have thought given the inventory of gloom and disapproval he faces," said Frank Graves, president of Ekos.

Darrell Bricker, president of polling firm Ipsos Canada, said this election is shaping up as the most difficult to predict in decades. The electorate appears volatile, he said, and his firm's research indicates households from most income classes are consumed with issues related to cost-of-living issues, such as higher grocery and hydro bills.

"There's a big preoccupation about making ends meet," he said.

Write to Paul Vieira at paul.vieira@wsj.com