Attention political junkies. Canada's two main opposition parties are on display this weekend, doing what comes most naturally to each one.

New Democrats gather in Montreal starting Friday to debate policy.

While in Ottawa, on Sunday, Liberals and their supporters meet to choose another leader — their fourth in less than a decade.

Convention coverage Peter Mansbridge, joined by political analyst Chris Hall, will host CBC News Network's special coverage of Sunday's Liberal leadership announcement, beginning at 5 p.m. ET. Evan Solomon will host CBC Radio One's Liberal leadership special, from 5:50-6:30 p.m. ET. All weekend long the latest breaking news, analysis and live-blogging from the conventions will be available at cbcnews.ca.

These competing conventions are a sign of what to expect from the NDP and Liberals as they mark the start of a two-year build-up to the next federal election, and the first of what will likely be many clashes for the attention of what both parties like to call ''progressive voters.''

As the Official Opposition, New Democrats start with the upper hand. But the Liberals do have history on their side, and a brand that, while battered, retains residual strength in many parts of the country.

What that means is there will be limited, if any, co-operation between the two in the next while.

No attempt to minimize the potential for vote splitting as the two parties compete with the Greens for the 60 per cent of Canadian voters who didn't choose Stephen Harper's Conservatives in 2011, not to mention for the growing number of Canadians who didn't bother to vote at all.

Insiders from both parties insist their only competition is the Conservatives.

Policies and personality

The truth, though, is that the NDP and Liberals are increasingly wary of each other. The signs are already there.

Take the NDP. The Orange Wave in Quebec came at the expense of both the Bloc Québécois and the Liberals in 2011. Those first-ever takeaway ridings may well prove difficult to hold the next time around.

This weekend's convention will no doubt try to continue the trend of placing the party more firmly in the political centre, where the vast majority of Canadians reside.

It is a positioning that began with the late Jack Layton, and has been continued by current party leader Tom Mulcair.

That means forget the resolutions calling on Mulcair to campaign for the nationalization of the energy industry, the big banks and insurance companies.

Instead, party strategists say look for the party to strengthen its commitment to responsible development of the oilsands, to protecting consumers from higher bank charges and to a free trade policy that balances opportunities for Canadian exporters while protecting supply management at home.

This isn't a sellout of the party's democratic socialist roots, it's an attempt at pragmatic policies intended to convince Canadians that the NDP can be trusted with the economy, and trusted not to make radical changes to the country's finances.

In other words, policies that the party can promote as a clear alternative to the Conservatives while leaving less room for the Liberals to stake a claim under their new leader.

"We need to distinguish the NDP from our political rivals in every part of the country,'' says one strategist. "In the West that means the Conservatives. But in the East, the Liberals can't be overlooked.''

But updating their economic policy is only one item on the New Democrats' to-do list this weekend.

The convention is also planning to showcase the more personal side of Mulcair, a side party strategists acknowledge Canadians really haven't seen during question period, or in the news conferences he's held around the country.

There are a number of events, starting Friday with an online Connect with Tom Mulcair to videos of him with his family and discussing his interests outside politics.

The party's confident Mulcair can make that personal connection with voters, while offering a contrasting image to the one created by media reports that suggest he is more Stephen Harper than Jack Layton.

It's a proactive response to the expected media onslaught that will begin Sunday when Liberals — barring some kind of misalignment of the cosmos — are expected to choose Justin Trudeau as their new leader.

The second coming

The 41-year-old Trudeau is the acknowledged front-runner and bearer of one of the most recognizable political names in Canadian history.

He also oozes win-ability, the least quantifiable of all concepts in politics, and the most easily undermined.

His policies remain largely unknown. His ability to hold his own against the more experienced Harper and Mulcair, unclear.

Trudeau did pen the occasional op-ed piece during the leadership race to outline some of his positions.

For example, he supported the oil patch purchase of Nexen by the state-owned Chinese company CNOOC as well as the controversial Keystone XL pipeline that would take Alberta's oilsands bitumen to refineries on the U.S. Gulf Coast.

And Liberal strategists are quick to note that Trudeau carries none of the baggage of his predecessors, that he has no connection to the sponsorship scandal in Quebec, for example.

He can also excite a crowd and, perhaps most importantly, this leadership campaign hasn't produced ammunition such as the infamous line ''You think it's easy to make priorities?'' that the Conservatives gleefully repeated in the attack ads that doomed former leader Stéphane Dion.

In this case, whoever wins inherits a new database of 300,000 supporters and members, people the party can, at the very least, solicit donations from in the future.

Even so, the challenges facing the Liberals as the third-place party in the Commons are considerable.

Can the new leader keep those new-found supporters engaged? Can the party re-establish itself in the West and rural Canada? And, most tellingly, will the Liberals be able to win back those voters who defected in the last election to both the NDP and the Conservatives?

So, starting Monday, the Liberals and the NDP begin anew, presenting their parties, their policies and their leaders as the best alternative to the government in the anticipated election two years from now.

Of course, the Conservatives will be doing what comes most naturally to them this weekend, too. Watching and waiting, ready to exploit any opening their opponents provide.