Frank Coleman, 60, a successful businessman based in Corner Brook, said he decided to leave public life because of a "significant and challenging family matter."
He would not elaborate at a news conference in St. John's, saying only that a challenge arose in the past week involving a member of his immediate family.
"My first and absolute loyalty is to my family and certainly not a lust for power," he said. "That is the one area of privacy that I have and it's going to be the one area of privacy that I keep."
Coleman, a political novice who has never held public office, was supposed to become leader of the party at a convention on July 5, then be sworn-in as the province's 12th premier shortly after.
"I feel bad for the position I've put the party in," he said. "It does have a bad look to it."
The province's interim premier, Tom Marshall, admitted the sudden turn of events was embarrassing for the party, though he insisted the machinery of government was moving along just fine.
"There's a premier and caucus in place with a mandate to govern," he said. "The government has not been affected at all. The PC party has been affected."
Marshall said a new leadership race would attract many candidates and could possibly be wrapped up at a convention soon after Labour Day.
The party's executive committee is expected to draft a plan at a meeting Tuesday night, Marshall said, adding he isn't interested in seeking the leadership.
"I said I would be here for the transition and that's it," he said.
Former cabinet minister John Ottenheimer publicly declared his candidacy Monday on radio station VOCM. He said he considered running for the leadership in the most recent race, but decided the timing wasn't right.
Coleman said that soon after he launched his leadership bid in March, the race took many "twists and turns" as one of his two rivals was kicked out of the race and a second dropped out — all within a month — leaving him the only candidate for the job.
"It was certainly not how I envisioned winning the race," he said.
However, he insisted he was not looking for an excuse to leave politics.
"I was in this to do it for all the right reasons. I have thoroughly enjoyed meeting the people of the province that I would love to serve."
The party has been in power since 2003 and began the search for a new leader in January when Kathy Dunderdale quit politics amid complaints about her leadership style and poor performances in public opinion polls.
Coleman has been under intense scrutiny since his wife Yvonne and other members of his family joined a pro-life march on Good Friday, as they have done for years. He later denied that his pro-life views would affect access to abortion.
He also had to defend the province's cancellation of a Labrador highway contract without any penalty that was awarded to Humber Valley Paving. He had previously led the company but has said he sold his shares last winter and resigned from the board of directors just before entering the leadership race.
Coleman has repeatedly denied that he personally gained from the contract cancellation negotiated at around the same time by his son. The provincial government has said it saved taxpayers money by not calling in related bonds before retendering the work that was slowed by Labrador forest fires.
The provincial auditor general is reviewing the deal at the government's request, a process that Marshall said would keeping going even after Coleman's departure.
Coleman's track record as head of the Coleman Group of Companies, which includes food, clothing and furniture retail businesses, saw him named Atlantic Canada's CEO of the year by Atlantic Business Magazine in 2010.
He became the sole leadership candidate in April when fisheries magnate Bill Barry quit the race, suggesting it was stacked against him. The only other challenger, retired naval officer Wayne Bennett, was expelled for breaching party principles.
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