TORONTO - Prime Minister Stephen Harper has laid out ground rules for any federal Tories thinking of wading into the Ontario Progressive Conservative leadership battle.

With the provincial leadership race showing signs of spilling into the federal arena, sources say the prime minister warned MPs at a caucus meeting last week that he did not want to see rifts in the national party.

Harper moved to pre-empt discord as leadership factions were already forming around his own cabinet table: Finance Minister Jim Flaherty's wife, Christine Elliott, is contemplating a run, and at least one other federal minister appears to be backing rival candidate Tim Hudak.

A Harper spokesman would not discuss details of the closed-door talk, but confirmed that federal Tories have been made aware of dos and don'ts.

"MPs, cabinet ministers are more than welcome to support whichever candidate they wish," and can even endorse their pick, said Kory Teneycke.

Their staff can campaign for a candidate -- just not during working hours, he said.

Anyone working in the Prime Minister's Office, federal Conservative headquarters, or in parliamentary research and communications is forbidden from getting involved in any leadership campaigns. They can play a neutral role, like ballot-counting, Teneycke said.

There has been speculation that some federal cabinet ministers with strong ties to the Ontario party were mulling a return to the provincial legislature, including Public Safety Minister Peter Van Loan.

Most have denied they're thinking about a run. Van Loan, who is in Washington this week to discuss border security with members of the Obama administration, is "very focused on his work as public safety minister," said his spokesman Chris McCluskey.

But it would be "difficult, if not impossible" for anyone without a provincial seat to convince the Ontario Tories that they should have the top job, said one party insider, who spoke on condition of anonymity.

"We've spent a lot of time on this particular issue and it's time to move on," said the Conservative, who is backing Hudak.

Outgoing leader John Tory, who will resign once an interim replacement is chosen next week, failed to win a seat in the 2007 provincial election, leaving him on the outside looking in for more than a year.

His drawn-out search for a provincial seat, which ended in a stunning byelection loss March 5, fuelled a rebellion among disgruntled party members who blamed Tory for the party's humiliating 2007 defeat.

A new leader is expected to be named at the end of June. While the rules of the race have not yet been finalized, the party's constitution suggests that candidates will have only a few weeks to get organized and sign up new members ahead of the crucial vote.

Tapping into the support network of a federal MP is one shortcut that's available to leadership hopefuls who want to hit the ground running, experts say.

"Different people in parties have different followings, and so I may be a Conservative MP, but I have a number of people who are loyal to me in my riding," said politics professor Henry Jacek at Hamilton's McMaster University.

"If I say, `I think we should go with X,' then the likelihood is that my followers -- to a large percentage -- will probably follow me because they are loyal to me."

The candidates will try their luck first with federal cabinet ministers who have influence in Ontario, such as Transport and Infrastructure Minister John Baird and Industry Minister Tony Clement, they say.

Immigration Minister Jason Kenney, who hails from Calgary, may also be wooed by provincial Tories anxious to gain access to his contacts in ethnic communities in the Toronto area.

He was invited to speak at the provincial party's convention last month, where he joked about having been made an "honorary" Ontarian.

Intense pre-campaigning has already begun, with Hudak, the party's right-leaning finance critic, quickly emerging as one of the favourites in the race.

Hudak, Elliott and Flaherty, as well as Randy Hillier -- who is also mulling a potential bid -- were all in Ottawa last week to attend a conference reception sponsored by the Manning Centre for Building Democracy, a conservative think-tank run by former Reform Party leader Preston Manning.

Provincial insiders say Hudak has the backing of former premier Mike Harris -- whose right-wing agenda made him a hero to the party's most fervent conservatives -- and his inner circle. Hudak is married to Harris's former chief of staff and served in his cabinet.

According to Conservative blogger Stephen Taylor, Hudak secured federal Justice Minister Rob Nicholson's blessing last week and may soon have Baird's.

Nicholson's spokesman Darren Eke wouldn't confirm the endorsement, but said the minister is "very supportive" of Hudak and has worked with him on a "variety of issues" over the years.

Another Ontario Conservative MP, Dean Del Mastro, invited fellow caucus members to a meet-and-greet with Elliott last Thursday in Ottawa, billing her as the finance minister's "significant other."

Tensions will inevitably build if Elliott and Hudak both run, but Harper won't tolerate any divisions in his caucus, said political scientist David Docherty of Waterloo's Wilfrid Laurier University.

"I think there may be some divisions that come out, but I suspect that we won't see them at play as long as Mr. Harper is leader of the Conservative party," he said.

"And I don't think people will probably be punished for their support one way or another as long as they kind of keep it off-line and not bring it up at caucus or cabinet."

Neither Hudak nor Elliott have formally announced whether they'll run, though Elliott confirmed she is "seriously considering" a bid and will make a final decision soon.

Elliott, a lawyer and mother of 18-year-old triplet sons who serves as the party's justice and women's issues critic, is considered to be a more centrist candidate than Hudak and may attract support from the party's so-called "Red Tories."

She acknowledged that some party members have felt "disenfranchised," but downplayed suggestions the party's neo-conservatives will push for a right-wing leader.

It's not a "question of left or right" anymore, but who can unite the party, said Elliott, who considers herself to be a "consensus builder."

"The next leader will need to certainly get everyone moving in the same direction and really bring that sense of energy to reinvigorate the party, because it has been a difficult time," she said.

Veteran caucus member Elizabeth Witmer has said she may take another shot at the job. Caucus chairman Frank Klees, who lost in the 2004 race that saw Tory take the crown, is widely expected to run as well.