Senator Marjory LeBreton is in her final days as Canada's substitute prime minister because when Stephen Harper shuffles his cabinet, she'll be bumped out of that role.

LeBreton is the government's leader in the Senate and has been dealing for months with the Senate expense scandal involving the housing and travel claims of Senators Patrick Brazeau, Mike Duffy, Mac Harb and Pam Wallin.

She announced last week that she is stepping down as Senate leader but didn't say when. The date is still to be determined. LeBreton intends to remain a senator; she is only quitting her job as the Conservatives' leader in the Senate.

But she's also vice-chair of an important cabinet committee and it's that position that makes her the number one minister who can fill in for Harper if the need arises. That's about to change.

Amidst the switching of portfolios, the identity of Harper's new backup is one more thing to watch for when the ministerial shakeup happens. Previous governments have had a deputy prime minister position but Harper did not maintain that tradition when he took power in 2006.

The Privy Council Office, the department that serves the prime minister and cabinet, maintains a list of ministers, in order of priority, who are authorized to substitute for Harper if he is unable to perform the functions of his office.

LeBreton, who is vice-chair of the cabinet's priorities and planning committee, is first on the list followed by Citizenship and Immigration Minister Jason Kenney, then Human Resources and Skills Development Minister Diane Finley, Defence Minister Peter MacKay, Treasury Board President Tony Clement, and Industry Minister Christian Paradis.

Kenney, Finley, MacKay, Clement and Paradis are all chairs of other cabinet committees. The priorities and planning committee, which is chaired by Harper, is the highest-ranking one. It guides the government's strategies, approves government appointments and carries out other functions.

Harper responded to LeBreton's announcement that she is stepping down as Senate leader with a statement that said he would take the summer to consult and would name a replacement before Parliament resumes Sept. 16.

LeBreton will be out of cabinet soon

He is expected to shuffle his cabinet in the coming days, however, and it is anticipated that he will make another break from tradition — no longer will the Senate leader be part of the cabinet.

That means LeBreton will be off the priorities and planning committee and therefore she will no longer be the person who would fill in for Harper.

The minister he picks to vice-chair that committee will be the new backup prime minister.

Bob Plamondon, a political author, historian and public policy consultant, says Harper may want to consider resurrecting the deputy prime minister job when he makes over the cabinet.

"This would be a way of delegating some of the authority and responsibility which I think would free up some of his time," he said in an interview after floating the idea on Twitter.

"From a political perspective, it would deal with the perception that he needs to be in command and control," he added.

LeBreton isn't the only one who announced in recent weeks that she's stepping down from a role. Earlier this week, former Public Safety Minister Vic Toews abruptly resigned as a Manitoba MP and cabinet minister. Who fills in for him?

The Privy Council Office has a list for that too. It has a backup for each minister and a second alternative and it shows that Justice Minister Rob Nicholson is the designated alternative for Toews.

Toews was Nicholson's backup but with him gone, the next minister on the list is Defence Minister Peter MacKay.

Here's a look at who else fills in for ministers in some key departments:

  • Finance: Jim Flaherty's backup is Treasury Board President Tony Clement.
  • Foreign Affairs: John Baird's backup is House leader Peter Van Loan
  • Industry: Christian Paradis's backup is Finance Minister Jim Flaherty
  • National Defence: Peter MacKay's backup is associate defence minister Kerry-Lynne Findlay.
  • Health: Leona Aglukkaq's backup is Treasury Board President Tony Clement.
  • Immigration: Jason Kenney's backup is Human Resources Minister Diane Finley.
  • Environment: Peter Kent's backup is Heritage Minister James Moore.