A surge in support for Canada's left-leaning New Democrats is forcing strategists and investors to consider a once unthinkable prospect — the party which has never governed federally might now win the October election.

The New Democratic Party (NDP) scored a stunning upset in Alberta's provincial election this month, ending four decades of Progressive Conservative rule. And after two years at a distant third in national polls it is now at or near the top.

The NDP still faces a formidable opponent in the governing Conservatives, who have held power for more than nine years. The party has a large war chest and Prime Minister Stephen Harper is an experienced campaigner.