OTTAWA (Reuters) - The shock departure of a Canadian cabinet heavyweight has fueled talk about how long Prime Minister Stephen Harper will stay in power, with some in his party predicting he is unlikely to serve a full term if he wins re-election this October.

Stephen Harper, Prime Minister of Canada, addresses the 69th United Nations General Assembly at the U.N. headquarters in New York September 25, 2014. REUTERS/Lucas Jackson

Harper, 56, who led his right-of-center Conservative Party to victory in early 2006, is seeking to pull off a rare fourth consecutive victory against a stiff opposition challenge.

Numerous party sources say if the Conservatives come first, but fail to secure a majority of seats in the House of Commons elected chamber, Harper’s days are numbered. Even if he wins a majority, which polls suggest is out of reach, some top party members expect he will be gone within two years.

“This is going to be his last election,” said one well-placed insider, who like others spoke on condition of anonymity.

These sources stressed no one was immediately seeking to depose Harper, who is respected inside the party for reuniting a fractured political right and returning it to office.

But two factors are at play.

One is that Canadian prime ministers rarely stay in power for more than a decade. By mid-2017 Harper would have been prime minister for 11-1/2 years, a point where voters may well want someone else to run what is one of the world’s major energy-exporting nations.

And privately, some Conservatives feel uneasy about the party’s polarizing style and say they are growing weary of Harper’s rigid control.

Harper is an uncompromising operator and Conservative legislators regularly accuse their opponents of being soft on crime and terrorism and harboring plans that would destroy the economy.

Aides to Harper - who deny any discontent in party ranks - say he is very mindful that the last Liberal government was undermined by a power struggle between the prime minister and finance minister.

TIGHT CONTROL

Harper makes many major policy announcements and even senior ministers are routinely required to clear their decisions through his staff. Indeed, when Foreign Minister John Baird abruptly quit in February, sources said he had tired of clashes with people in Harper’s office.

Harper suffered another blow last Friday when Justice Minister Peter MacKay, a popular politician with the common touch Harper lacks, said he would quit after the election to spend more time with his young family.

If Harper wins without a parliamentary majority, opposition parties will most likely form a coalition to bring him down and few Conservatives believe he would stay on then as party leader.

The left-leaning New Democrats surged in the polls after winning Alberta’s provincial election last month and together with the opposition Liberals look to have enough support to deprive Harper of a majority.

But even if Harper extends the Conservatives’ run, party sources said he might only have another two years. Under this scenario, he would announce in mid-2017 plans to step down, giving his successor a year to 18 months in government before the next election in 2019.

To be sure, Harper could decide to run again. Farm Minister Gerry Ritz, asked whether he thought this was Harper’s last election, replied: “I hope not. He’s still got lots to give.”

But Harper would probably face private pressure from legislators to go in mid-2017 on the grounds that it was time for change, say well-placed Conservatives.

A senior government source said Harper was focusing on the October election.

“If re-elected ... the prime minister has every intention of serving out a full term,” said the source.

If Harper loses in October, Canada would most likely take more serious steps to combat global warming.

The Conservatives infuriated greens and opposition parties by pulling Canada out of the Kyoto protocol on climate change and Harper has resisted the idea of either imposing a carbon tax or unilaterally regulating emissions from the oil and gas sector.

With potential leadership candidates Baird and MacKay now gone, the way seems clear for Defence Minister Jason Kenney, 47, who is popular among local Conservative organizations.

Kenney is the unofficial leader of the party’s socially conservative wing, which has long chafed about Harper’s refusal to touch issues such as gay marriage and abortion on the grounds they could alienate swing voters.

When pressed about potential leadership ambitions, though, Kenney says he already has a big enough job.

Harper faces no immediate threats, since Canadian leaders are elected by national party conventions - typically held every two years - rather than legislators.

Insiders note he has achieved much of his agenda, such as cutting taxes, boosting the military, pushing for tougher sentencing laws and signing free trade deals.

By mid-2017, he would be Canada’s fifth longest-serving prime minister and those who have worked with him say he might be ready to leave then.

“I think he’s bored ... He has a huge intellectual capacity. He is determined to accomplish things,” said a former top Conservative official, suggesting Harper might take on an international role.