If you want change, don’t risk your vote.

All polls show a majority of Canadians want to use their vote to defeat Stephen Harper. But with parties so close in opinion polls, many voters don’t know which opposition leader to support to get the job done.

Let’s look at this two ways, starting with electoral math. There are 338 riding elections across the country. Tom Mulcair starts with almost 100 incumbents, Mr. Trudeau only about 35. Advantage NDP.

And the NDP vote is more efficient. Based on current levels, Tom Mulcair’s NDP will win 60 or so seats in Quebec and another 20 or 25 in BC. This regional concentration explains why riding projections consistently show the NDP leading in seats even at similar levels of national support. Again, advantage NDP.

So based on electoral math, Tom Mulcair is the closest to defeating Stephen Harper.

But there’s another critical problem with voting Liberal to defeat Stephen Harper. At current support levels, no party is winning a majority.

Tom Mulcair has been clear his goal is to defeat Stephen Harper – with an NDP majority if voters give it to him, but by working with the Liberals if necessary. NDP voters are assured their vote will never be used to prop up a Stephen Harper minority.

Liberals voters can’t be sure. The Trudeau Liberals may well prop up a Stephen Harper minority.

If you don’t think he’d do it, remember he’s done it before. Justin Trudeau’s Liberals propped up Stephen Harper’s minority after the 2008 election. Mr. Trudeau voted for Harper’s budgets and the cuts Trudeau says he’s against now. And unlike Tom Mulcair, Mr. Trudeau won’t commit to working together to defeat Stephen Harper.

A Liberal vote is a risk we can’t take if we want to defeat Stephen Harper and bring change to Ottawa.

Truth is, given all of Mr. Trudeau’s reversals, it really is impossible to know what Mr. Trudeau would do after the election.

Mr. Trudeau broke a promise to his party members to hold open nominations. What followed were Trudeau-preferred candidates, meddling and lawsuits.

Then this spring Mr. Trudeau voted with Stephen Harper on Bill C51 – even through Mr. Trudeau himself could not say if the bill violates Canada’s Charters of Rights and Freedoms.

Mr. Trudeau now campaigns against raising the federal minimum wage to $15, even though he voted for it in the Commons. He broke his promise to over 100,000 of the lowest-paid workers in Canada.

Mr. Trudeau promised a fully-costed campaign platform, then tried to say he’d already given it and now -- under pressure from journalists -- he says he’ll provide the cost details.

Mr. Trudeau now opposes a childcare plan. More broken promises—this time to young moms and dads who want help with everyday affordability.

And then the strangest reversal. Until recently, Mr. Trudeau professed faith in balanced budgets as a core Liberal value. But then he had a sudden conversion – now deficits would be a core Liberal value!

At one moment in last week’s debate -- between his constant interruptions --Mr. Trudeau said he was being honest. Well, Mr. Trudeau, on open nominations, C51, childcare, costed platforms, minimum wage and balanced budgets -- were you being honest when you supported them? Or when you opposed them? Pick one, it can’t be both.

For the 70% of Canadians who want to defeat Stephen Harper and bring change to Ottawa, Tom Mulcair is a reliable choice. Mr. Trudeau – not so much.

- Parkin is a frequent NDP media commentator.