OTTAWA (Reuters) - With Canada’s three main political parties all getting around 30 percent support in polls, the odds of a period of political instability after the October 19 election are rising.

L-R: Conservative Leader Stephen Harper, NDP Leader Tom Mulcair, Green party Leader Elizabeth May and Liberal Leader Justin Trudeau take part in the French-language debate in Montreal September 24, 2015. REUTERS/Adrian Wyld/Pool

It could also shed an awkward spotlight on the constitutional role played by Governor General David Johnston, who is Queen Elizabeth II’s representative in this member of the British Commonwealth.

A proposal from Canada’s Green Party leader Elizabeth May for the two main parties on the left to take power immediately if Conservative Prime Minister Stephen Harper fails to get a majority - even if his party has more seats than anyone else - has triggered concerns that Johnston will face a very difficult decision.

While few see a full-blown constitutional crisis erupting, investors and economists warn that prolonged political uncertainty could undermine Canadian financial markets at a time when the economy has seen two straight quarters of negative growth, for many the definition of a recession, as the nation’s energy sector suffers from plunging oil prices. The Canadian dollar is already trading at near 11-year lows.

Johnston, 74, who is appointed by the Queen on the recommendation of the prime minister, is usually a figurehead who presents awards and attends commemorative events while also giving routine royal assent to Canadian laws once they pass Parliament. But after an election, he plays a critical role by asking one of the political leaders to form a new government.

This is easy if one of the parties gets a majority of the 338 seats in Parliament. It gets a lot trickier if they are all short of that, and there is the possibility of a coalition being formed between parties, or at least some kind of cooperation that would allow one to govern with the other’s support.

“There could be a lot of confusion and disagreement and competing claims, and it’s not clear how those would get sorted out,” said Mark Jarvis, from Toronto-based think tank the Mowat Centre. “In a worst case scenario, we could potentially have a great deal of confusion about who has a legitimate claim to form a government.”

COULD FAIL QUICKLY

If Harper’s Conservatives get more seats than his two opposition left-of-center rivals, the Liberals and the New Democratic Party, then by tradition, the governor general would be expected to ask Harper to form the next government. Harper has said the party with the most seats should govern.

But some constitutional experts said the Greens’ proposal could be a viable alternative and reduce instability.

The danger if Harper forms a government without a mandate in seats or votes is that it could fail quickly.

He would likely lose the first vote in the House of Commons after the election, which would be a vote on the so-called Speech from the Throne, in which the new government lays out its policies. Its rejection would show that Harper would find it next to impossible to govern effectively.

May proposes that Johnston not wait for that but instead ask the Liberals and the NDP to form a government if they are prepared to support each other. She says the Greens, who have about 4 percent support in polls, wouldn’t join a coalition themselves.

Certainly, there is a good chance that this election will create historical waves. A Canadian prime minister has never been defeated on the first throne speech after an election, and Canada last had a coalition government in 1917.

Edward Schreyer, who was governor general from 1979-84, said if a combination of parties could show they could form a coalition with majority support, they would be allowed to form a government. “What the country needs, what the governor general must look for, is evidence of a stable government,” he said.

But insiders in all three main parties said if Harper had the most seats he would have the right to form a government and present his policies at the throne speech, usually several months after the election.

‘SNOWBALL’S CHANCE IN HELL’

Asked by Reuters whether the Liberals would back a Harper minority government, the party’s leader Justin Trudeau on Tuesday said at an event in Montreal: “There are no circumstances in which I would support Stephen Harper to continue being prime minister of this country.”

If the opposition brought down the government in a confidence vote over the throne speech, then the governor general would have to decide whether to let the second-place party try to form a government. He could also call a new election if Harper asked for one.

While up until now Trudeau has rejected the idea of a coalition or a formal arrangement with the NDP, he did indicate last week an openness to backing an NDP minority government, saying his party “has always been open in minority situations to working with other members of the house to pass legislation that serves Canadians.”

NDP leader Thomas Mulcair said last Wednesday there was not “a snowball’s chance in hell” of supporting a Conservative throne speech. He has said he is open to working with parties other than the Conservatives but says Trudeau has slammed the door shut on cooperation.

A lack of clarity on who was in the driver’s seat “is negative for GDP growth and unemployment,” said Canadian economist David Madani at Capital Economics.

Paul Taylor, chief investment officer of BMO Asset Management Canada, said a negative market reaction was guaranteed if there was no clear winner. “Anything Canadian should come off a little - the Canadian dollar, Canadian bonds and Canadian equities,’ he said.

STICK TO THE PRINCIPLES

Johnston, a constitutional expert himself, advised then Ontario Lieutenant Governor David Onley in 2014 when he looked set to face a similar constitutional problem in the province.

Onley said Johnston told him to “just remember, you stick to the basic principles ... (those are) the sitting prime minister or sitting premier has the right to not resign even though his party may have fewer numbers of seats than one or both of the other parties.”

Johnston’s office said in a statement that while various scenarios can arise if no party wins a majority, it “will not hypothesize on potential courses of action” he may take, adding it was inappropriate to speculate on what he would do if party leaders called him about other power-sharing arrangements.

More recently, Harper fended off an attempt in 2008 by a Liberal-NDP coalition to bring down his government, calling it an undemocratic power grab.

Facing a confidence vote that would have defeated his minority government and seen the coalition take office, Harper bought time by asking and getting then-governor general Michaelle Jean to suspend Parliament.

One of her advisors, Peter Russell, told OntarioNewsWatch.com in 2012 that Jean feared “a dreadful crisis” if she had allowed the coalition to form a new government and then the Conservatives had condemned the decision as equivalent to a coup d’etat. “We would have been there in the headlines of the world like Greece. (That’s) not very good for the country in any which way.”

Jean, who is now secretary-general of the Paris-based Francophonie organization, declined comment.

One option at the time was to ask the Queen to dismiss or overrule Jean if she had refused to suspend Parliament, a Harper spokesman Kory Teneycke said in the 2010 book “Harperland.” Teneycke, who is now Conservative campaign spokesman, later said his comments were taken out of context.

Experts said there was little chance of the Queen wading into Canadian politics.

“There’s no way that the Queen is going to get dragged in,” said Robert Hazell, director of the Constitution Unit at University College London. “Even if he (Harper) were to appeal to the Queen, Buckingham Palace would refer it straight back to the governor general.”