If the practice of politics is famously known as the art of the possible, the historic success of the New Democratic Party in last year’s federal election was a masterpiece of the oeuvre: the perennially minor parliamentary player smashed conventional wisdom with an astonishing breakthrough that the smart money had always rejected as unlikely, if not impossible.

But it didn’t happen by accident. Millions of Canadians who had never before supported the party didn’t make the switch just because of the late Jack Layton’s famous smile. In Quebec, ground zero of this political earthquake, it took years of largely unseen preparation, long nights of hard work and a series of carefully calibrated political appeals to make this seismic political shift possible.

Now, as the almost 130,000 members of the federal NDP begin to make their choice for the person to replace Jack Layton as leader, the stakes have never been higher. Whoever emerges as the Leader of the Official Opposition on March 24 will assume a level of responsibility that goes far beyond internal party politics.

As we have seen since Stephen Harper’s Tories narrowly won their coveted majority only 10 months ago, our democracy may not survive the ordeal. There is no point in pulling punches here: It is imperative that the Conservatives be defeated in the next election, or, at the very least, be reduced to a weak minority.

Who, among the seven remaining candidates, is best able to accomplish this mission? Will NDP members endorse the choice of the party establishment, Brian Topp? Or will they choose the slick Johnny-come-lately to social democracy, the former Quebec Liberal cabinet minister Thomas Mulcair? Yes, there are other valid, credible candidates in the race. But these two largely represent what some are portraying as an epic struggle for the soul of a party that has long prided itself on not selling out its values in a craven quest for political power.

The conceit is not completely true. Not many prominent New Democrats are still campaigning to nationalize the country’s banks, for instance. But there is an increasingly nasty effort to portray Mulcair as unworthy of the NDP leadership; that he is essentially a Trojan horse who would betray the party’s dearest principles.

For myself, this is not an abstract question. Regular readers of this column know where I stand on the political spectrum. The NDP’s leadership race is even more personal for me since for the first time I now have a vote: I could no longer stand on the sidelines delivering more-or-less detached analysis and critical commentary (which happen to be among my favourite activities). This time I feel my duty as a citizen is to engage, to assume the responsibility of our democratic privilege at a time it is threatened as never before in the modern era.

But I hesitated. Many of the people in the party I greatly respect depict Mulcair’s candidacy as a heresy to the cherished social democratic principles that I share. Some, the “ABM” folks, got involved in Topp’s campaign simply as an effort to stop Mulcair.

There are caveats to be sure. He is blunt on saying he would not reopen Canada’s free trade agreements, something I have long felt to be an important goal for any future progressive government. He waves away the popular party position of raising taxes on the wealthy as a needless political handicap. On foreign policy, he unabashedly supports Israel, which is a prima facie failure of the litmus test among the NDP left.

Despite this, I have come to the conclusion that Thomas Mulcair is the only candidate who has any shot at defeating Stephen Harper’s Conservative government. And that is the highest priority. I don’t have to worship the party leader to work for the health of my country.

Romeo Saganash, after his recent withdrawal from the leadership race, has come to a similar conclusion. His endorsement of Mulcair on March 7 had a sobering effect on the campaign. And that’s something that I am noting: Brian Topp’s biggest backers jumped on his bandwagon last summer before the race was really under way. Mulcair’s biggest endorsements are coming near the end of the campaign after all the candidates’ strengths and weaknesses have been well exposed and evaluated.

And that, despite my initial misgivings and policy disagreements, is why I will vote for Thomas Mulcair as NDP leader. He is the only person in this race who I feel can create the possibility of progressive change in this dark political era.