OTTAWA — He was chosen for his political neutrality, his academic prowess and his common sense.

But when David Johnston becomes governor general on Friday, Canadians will soon discover he might also need nerves of steel and a diplomat's touch as he guides this country through a political minefield and tries to avert a constitutional crisis.

Although much of his job will be ceremonial and symbolic, constitutional and parliamentary experts agree on one thing: Johnston could be the one man in Canada who ultimately decides who forms the government if the next election produces another minority Parliament.

Such power won't be exercised lightly or without political controversy, the experts predict. But they also say Johnston has what it takes to make the right call.

"He's an intelligent man and he doesn't get easily rattled," says Edward McWhinney, a constitutional law expert and author.

"He's had a lot of experience. He's a pleasant, personable man. As far as one can see, he has no nervous qualities that might undermine him during a crisis. In other words, I think he's got calmness and common sense."

Queen's University professor Ned Franks, one of Canada's foremost experts on Parliament, says Johnston could be at the centre of a political maelstrom after the next election if the new government is toppled in Parliament.

"It leaves him having to make a decision that affects who governs Canada. Somebody has to make the decision."

Franks says none of this would necessarily be easy for Johnston.

"It takes great courage, and the capacity to live with yourself. Because you're making one of those decision that we all make where you don't know the consequences but you make the best choice."

Johnston was chosen by Prime Minister Stephen Harper after a six-person advisory committee quietly held consultations earlier this year in which constitutional experts, past and present political leaders and others were canvassed for their views.

Harper's office says the committee members were guided by "one key question" as they delivered their recommendation to the prime minister: will the next governor general be able to serve without partisanship?

In that regard, Johnston was an inspired choice. He is a long-standing scholar with legal expertise who has served on corporate boards and has also provided advice to governments. The closest he has come to the political sphere was in defending federalism against Quebec separatists and in moderating televised leaders' debates in federal elections.

He'll need the reputation of neutrality.

Johnston becomes governor general at a particularly unusual time in Canadian politics. Minority governments have become the norm. The odds of either Harper's Conservatives or Michael Ignatieff's Liberals winning a majority in the next election are slim.

That political reality has given birth to the following scenario: Canadians go to the polls, possibly next spring, and Harper wins another minority government. His government is quickly defeated in the House of Commons on a vote of confidence. Harper goes to Johnston to seek the dissolution of Parliament and request another election.

But Ignatieff, after having had discussions with one or both of the other opposition parties, requests Johnson's permission to form a government with the formal or informal support of the NDP and/or the Bloc. Harper objects, citing catastrophic risks to the economy and national unity from what he calls a "coalition."