OTTAWA—The two men who would unseat Stephen Harper face two very different challenges this summer.

Liberal Leader Justin Trudeau is about to discover how difficult it can be to regain political momentum once you’ve lost it.

NDP Leader Tom Mulcair will learn it is often easier to claw your way to the top of the political heap than it is to stay there.

If by Labour Day, the two have settled into a familiar pattern of splitting the anti-Harper, so-called “progressive” vote, that equilibrium will likely lead to the re-election of the Conservatives.

So, the real summer skirmish for political-watchers will be between Trudeau and Mulcair.

Right now, Mulcair is making a far more compelling case as the real threat to Harper, but with that status comes increased scrutiny.

How he handles that scrutiny — from the media, from the voters and from his political opponents — will be key. Despite an embarrassing misstep in which the NDP leader appeared at a loss on this country’s corporate tax rate, Mulcair appears ready to handle that scrutiny.

Canadians may have seen a preview of the campaign when Harper and Mulcair went mano-a-mano in the last question period attended by all three leaders before the campaign.

The dynamic was fascinating — while those two men duked it out on a series of issues, Trudeau was a spectator. This is precisely the dynamic the NDP wants to take on the campaign trail, distilling this battle into a left-right struggle with the incumbent while squeezing Trudeau into irrelevancy.

To protect themselves heading to October, the party has taken some smart, pragmatic steps.

Don’t expect a budget from Alberta Premier Rachel Notley, for example, until after a federal vote, reducing the odds of any NDP buyer’s remorse in that province.

Mulcair laid out much of his election platform early in a bid to allay fears of a radical agenda. He is now preaching a more moderate version of “change,” than Trudeau, who is campaigning under the slogan of “Real Change.”

A string of Senate scandals has been a never-ending gift for the party, but the NDP knows Senate reform will not swing an election so Mulcair will tack close to the flame — fanning populist disgust with the institution, but pulling back in time to avoid being painted as a potential prime minister selling constitutional logjams.

That only works, however, if his opponents let him pull back and they know there is no way to open the constitution to a single issue. First Nations and the Bloc Québécois, under the new (old) leadership of Gilles Duceppe will see to that.

Alleged spending irregularities by the party do not receive nearly the attention of spending irregularities in the Senate, but the longer he constitutes a threat, the louder Conservatives will shout about a satellite office controversy which hasn’t really busted out of the Ottawa bubble.

The Board of Internal Economy, which New Democrats delight in labelling a “kangaroo court,” has ordered 68 current and former NDP MPs to repay $2.75 million for the satellite office scheme plus $1.2 million for misuse of parliamentary mailing privileges.

Mulcair maintains his party has played by the rules.

Harper has his woes. He has now lost his two key political players from both coasts and conspiracy theorists may want to look at the gala send-off from the prime minister to Justice Minister Peter MacKay in Nova Scotia and the lack of even a formal statement wishing Industry Minister James Moore of British Columbia well.

For his part, Trudeau will continue to lay out policy planks, beginning here Monday when he wades into the Canada-U.S. relationship. He is expected to characterize it as the worst in at least two generations, a deep freeze he will blame on Harper and his inability to co-operate with anyone of a different ideological stripe.

Trudeau will also tell an audience our neglected relationship with our third NAFTA partner, Mexico — notable because of an unnecessary visa spat initiated by Canada — has further cost us influence in Washington, where the stalled Keystone XL pipeline is emblematic of a lost opportunity on continental energy and environmental policy.

Trudeau will also roll out environmental policy in Vancouver at the end of the month, flesh out an urban infrastructure program, unveil immigration policy and deliver his aboriginal policy to the Assembly of First Nations next month.

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Trudeau must find a way to bust back into the national conversation and do it during the summer months.

He’ll find that chase for the lost momentum daunting because merely catching Mulcair will not be enough. That would allow Harper to play vote-split bingo again in a return to power. Voters will understand that.