Shortly after Parliament returns next week, the Harper government will introduce a new anti-terrorism bill. One purpose of that bill will be to wedge Justin Trudeau on security. This is bad news for Mr. Trudeau.

A wedge issue is one in which Politician A takes a clear stand on one side and Politician B takes a clear stand on the other, causing some of B's supporters to switch over to A because the issue is so important to them. Stephen Harper has been seeking a wedge against the Liberals ever since Mr. Trudeau became leader, but without success.

He thought he might have it on trade, after Canada negotiated an agreement with the European Union. But Mr. Trudeau stood in the House and congratulated Mr. Harper on the accord. Trade will not be an issue in the election.

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He thought he might have it on foreign policy, when Israel entered Gaza in response to rocket attacks, or when Ukrainians rose up against their Russian-dominated government, but Mr. Trudeau was every bit as bellicose as the Prime Minister on both issues.

Then Islamic State arose in Syria and Iraq, and Mr. Trudeau made the wrong call.

Security is the right-wing equivalent of the environment. Both are event-driven. A terrible hurricane or forest fires caused by drought will increase voter concern over global warming. But if the climate is behaving, environmental concerns take second place to economic concerns.

The same is true of security. A terrorist attack, especially one close to home, will have people worrying about what their governments are doing to keep them safe. But as the memory of the latest bombing or shooting fades, so does the fear.

Politically, Mr. Trudeau had sound reasons last October to oppose the government's decision to join the Americans and Europeans in fighting Islamic State. Mr. Trudeau supports a more Pearsonian, peacekeeping-based, bridge-building (rather than bombing) approach to foreign affairs, unlike what he sees as Mr. Harper's bellicose solidarity with the United States.

Jean Chrétien's decision not to support the American-led invasion of Iraq was popular at the time he made it, and even more popular once he was proven right.

Foreign excursions never go down well in Quebec, where Liberals are determined to make gains against the NDP, who also oppose the mission. On the whole, it must have seemed like a good idea at the time.

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But Mr. Trudeau went further, dismissing the Canadian mission in Iraq as "trying to whip out our CF-18s and show them how big they are."

And then came the killing of Warrant Officer Patrice Vincent in Quebec and Michael Zehaf-Bibeau's attack on Parliament Hill. And then came the attacks in Paris. Mr. Harper's claim that "the international jihadist movement has declared war" on the Western democracies seemed entirely accurate. Mr. Trudeau seemed guilty of flippant passivism in a face of a dangerous threat.

The Liberal Leader can only hope – we can all only hope – that future threats are few and thwarted, and the security issue recedes in importance in voters' minds.

But the government will do everything in its power to keep the issue alive. There will be the privacy-versus-security debate over the anti-terrorism legislation. Then, in the first week of April, the mandate for the Canadian mission in Iraq will expire. Mr. Harper will certainly announce its extension, and ask Parliament to endorse his decision. The Liberals will have no choice but to continue their opposition.

Assuming the extension lasts another six months, the next date for renewing the mandate will occur in the first week of October – right in the middle of the election campaign.

In that campaign, Mr. Harper will remind voters that his government has fought Islamic extremism in Afghanistan, in Iraq and at home, and with the new anti-terrorism law. He will tell voters that Mr. Trudeau prefers crude jokes to tough decisions. He will seek to make security a major issue, and the chances are good he will succeed.

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It may not be enough to swing the election: hope and change may trump jobs and security. But it's the kind of ground on which Conservatives love to fight.

Mr. Trudeau may come to wish he had not opposed the mission in Iraq. And he will certainly wish he had never made that joke.