It's often said that timing is everything in politics. Scott Brison, the long-time Liberal cabinet minister with Conservative roots, has decided it's time to bow out.

He spent his farewell round of political interviews on Thursday defending the timing of his departure, coming as it does following allegations of political interference related to the criminal case against the military's former second-in-command, Vice-Admiral Mark Norman.

Prime Minister Justin Trudeau canvassed his team last summer asking who was sticking around to fight the upcoming election, according to multiple Liberal sources. At the time, Brison's name was on that list of those who were staying.

Since then, however, there's been a steady drip of politically toxic allegations emerging from the case against Norman, the former vice chief of the defence staff charged with one count of breach of trust and accused by the Crown of leaking cabinet secrets.

His defence team has sought to drag Brison in because he was instrumental in the leak-prone cabinet meeting at the heart of the case. Norman is accused of leaking to the media the Liberal government's decision at that meeting to put a plan to lease a supply ship for the navy on hold.

Norman's lawyers alleged he tried to torpedo that $668 million leasing contract with the Davie shipyard in Levis, Que. on behalf of rival Irving Shipbuilding in his home province of Nova Scotia.

On Thursday, Brison denied — vehemently and more than once — that the case had anything to do with the timing of his decision to end his two decades in federal politics.

"Absolutely not. That's a non-factor," Brison told CBC News Network's Power & Politics in a interview. "That's a matter between the prosecution and Vice Admiral Norman. This is a family decision and my family comes first."

Treasury Board President Scott Brison joined Power & Politics Thursday to discuss his decision to leave politics and what he plans on doing next. 8:10

After each salvo in the Norman case came behind-the-scenes news reports suggesting that Brison would not run in 2019 — rumours that the people around him were quick to pour cold water over.

Brison's name had long been linked, behind the scenes, with the Norman case, from the moment the former commander of the navy was abruptly suspended in January 2017.

It burst out into the open when Norman's lawyers, led by Marie Henein, went to court in October to force the federal government to release documents related to the case.

Relying on extracts from RCMP interviews — statements that had not been tested in court —​ Henein alleged Brison had close ties with the powerful Irving family.

'Engaged in transactions​'

Brison denied that claim. Under repeated opposition questioning in the House of Commons last fall, he said his presence at the cabinet committee in question, on Nov. 15, 2015, was part of his job as the minister guarding the public purse.

"My mandate as president of the Treasury Board is to ensure due diligence in the expenditure of public funds and to perform a challenge function, particularly in terms of the procurement function," he told the House of Commons on Oct. 15.

A contradiction emerged just over a month later, at the end of November, when more partial police transcripts were released.

Brison was quoted as telling the Mounties that Treasury Board was "engaged in transactions" related to the shipbuilding file, but he wasn't at the meeting for that reason.

"This wasn't my role as … as a member of this committee," he told the RCMP.

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Those rumours persisted throughout the fall up to the five-day hearing in mid-December where Norman's lawyers argued with the Crown and federal lawyers about the disclosure of documents, including Brison's emails related to the shipbuilding deal.

The federal government is in process of handing over thousands of pages of documents. A judge will determine if they are relevant to the case.

On Dec. 14, it was revealed that Brison and former defence minister Peter MacKay were on the Crown's list of potential witnesses for the trial, slated to begin in August — just as Canadian are getting ready to go to the polls.

The timing could be a coincidence — but just days later, Brison broke the news to his boss.

"Just before Christmas, I informed the prime minister of my decision not to run again in 2019," Brison said in an interview Thursday with CBC News.

"He was surprised and we had a conversation and when it became obvious that I wasn't going to change my mind, he totally got it."