If it were not for Peter MacKay, Stephen Harper would never have become prime minister and it is an open question whether he can secure another majority mandate or, for that matter, another election victory, without him.

What is certain is that, in the absence at his side of the man who led the Progressive Conservatives to his tent a bit more than decade ago, Harper’s task of leading his party to a fourth mandate this fall has become more daunting . . . and a bit more lonely.

If you believe, as I do, that the moderate conservative voters who gave Harper a majority in 2011 stand to decide the outcome of the upcoming campaign, then the loss of MacKay as a candidate is a grievous one.

Preventing red Tory voters from turning into blue Liberals (or taking on a shade of NDP orange) will be harder in his absence.

The first immediate consequence of the minister of justice’s decision to not run again is to make the Conservatives more vulnerable in Atlantic Canada, a region where they were already facing a steep uphill battle to hang on to the 13 seats they currently hold.

Justin Trudeau’s Liberals hold a commanding lead across the four provinces.

Mackay — whose political base is Nova Scotia — has always stood taller than his boss in a region where the word progressive is still deemed to have a place on the conservative label. There is not, on Harper’s cabinet or backbench, another Atlantic Canada figure of comparable stature.

MacKay was elected to the House of Commons under Jean Charest in 1997. He served under Joe Clark. His father, Elmer, was a fixture in Brian Mulroney’s governments. As Tory leader, the justice minister was the co-builder of the bridge between the Progressive Conservatives and the Reform/Alliance branch of the Canadian right.

But that 2003 bridge has seen more Progressive Conservative traffic headed out of the government’s command centre than going in the opposite direction over the past few years.

Senate speaker Pierre-Claude Nolin passed away earlier this year. Ontario senator Hugh Segal has moved on to academia. They were among the last leading red Tory establishment members on Parliament Hill.

Over the Harper decade the influence of the Tory side of the Conservative family has steadily declined.

The regiment of Progressive Conservatives that the justice minister was purported to lead from his senior post in Harper’s government is by now mostly outside the power loop looking in.

It remains to be seen whether they will be as motivated to mobilize behind Harper’s re-election bid this fall as they could be in assisting MacKay in a future leadership bid.

MacKay’s upcoming retirement may not change the immediate directions of the government but it could alter the political dynamics surrounding the looming legislative debate over end-of-life care.

After the Supreme Court struck down the prohibition on medically assisted suicide earlier this year, it was MacKay as justice minister who vowed that the government would not use the notwithstanding clause of the Constitution to circumvent the ruling.

Harper is under pressure from the religious right to do just that. With MacKay out of the picture, it is unclear whether the ruling Conservatives — if they are re-elected to government next fall — will feel bound by the assurances that he gave.

His imminent departure from politics tilts the balance of influence a bit more toward the social conservative wing of Harper’s caucus. Their cabinet champion, national defence minister Jason Kenney, is fast outliving his competition for insider influence.

With the exception of industry minister James Moore, none of the other major architects of Harper’s three election victories will be around next fall to fight alongside Harper in what is lining up to be his most challenging campaign battle since 2 004.

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Jim Flaherty is no more. John Baird is off to a lucrative private sector career. After his defeat in Alberta, Jim Prentice is a spent quantity. None of their cabinet successors has emerged as a force to be reckoned with on par with those former ministers.

On the eve of his fifth election campaign as leader, Harper more than ever stands head and shoulders over his cabinet team but that is more by default than anything else.

Chantal Hébert is a national affairs writer. Her column appears Tuesday, Thursday and Saturday.

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