New rules set out by the Liberal Party say an interim party leader won't be allowed to run for the party's long-term leadership and won't be able to talk about a merger with the NDP, according to an internal party document obtained by CBC News.

The party's executive is expected to pass the rules at its meeting Monday night.

The document says any candidate for interim leader will be expected to agree in writing that "he/she will not seek the permanent leadership of the Party as part of the next leadership selection process," or  "engage in any discussions or negotiations that would require any fundamental or material change to the nature or structure of the party," without prior approval done by a vote at a party convention.

The new rules also say the election of an interim leader will require the votes of a majority of MPs, and not only a majority of caucus, which includes more senators than sitting members of the House of Commons.

There are currently 46 Liberal senators and 34 MPs, with one senator to retire this week. More than half the senators — 29 — were appointed by former prime minister Jean Chrétien, who has already demonstrated his interest in the race by calling at least a half-dozen MPs to suggest Bob Rae would make a good leader.

The diminished Liberal caucus will deliberate the interim leadership Wednesday and choose someone to temporarily replace Michael Ignatieff at a May 18 meeting.

The party's leadership, known as the national board, will meet May 19 to appoint the interim leader based on the caucus's recommendation.

Ignatieff announced his resignation as Liberal leader the morning after the May 2 election when his party lost Official Opposition status to the NDP and was reduced to 34 MPs, down from 77. Ignatieff also lost his own seat in the House of Commons, defeated by Conservative Bernard Trottier in Etobicoke-Lakeshore, and has already accepted a teaching job at the University of Toronto.

Ignatieff's resignation immediately prompted speculation about who should be tapped to take over, and raised questions about whether the interim leader would also be eligible to run in a leadership race for the permanent job. It's a convention, but not a rule, that an interim leader only takes the job temporarily.

A permanent leader will be chosen at a party convention, and the party has yet to decide when that will take place. If the timeline set out in the party's constitution is followed, a new leader would have to be chosen within about six months but it's possible an amendment may be made to the constitution to allow for a convention to be held later.

The draft statement outlines a proposal to get around the constitution.

It suggests that the national board convene an extraordinary "special convention" on June 12, conducted via the internet, to amend the constitution. At that virtual convention, delegates would be asked to approve an amendment that would defer a leadership vote to some time between May 1, 2012, and June 15, 2013, with the exact date to be fixed at the discretion of the national board.

Delegates would also be asked to postpone the party's biennial convention, currently slated for December this year, to Jan. 13-15, 2012.

Many Liberals, including former prime minister Jean Chrétien, are saying there is no rush to choose a new leader after an interim one is chosen.

"If there were to be a delay, it would be a good thing, between the interim and the leadership," he said Monday following a speech in Quebec City. Because the Conservatives won a majority government on May 2, another election won't come for four years, and Chrétien said that gives the Liberals a lot of time to make its decisions. "We don't have to rush," he said.

But Chrétien wasted no time last week wading into questions about the leadership and future of the party. He was making calls to re-elected MPs, offering congratulations and in the course of some conversations, he was urging them to support Bob Rae as interim leader of the party. On Monday, however, he held back from openly promoting him or any other possible contender for the job.

Chrétien: Up to party to pick leader

Rae hasn't said yet whether he wants to run for the leadership job, either on an interim or a permanent basis. Chrétien called at least half a dozen MPs and was pushing Rae, a former leadership contender against Ignatieff in 2006 when Stéphane Dion ended up winning the job. When he asked about supporting Rae on Monday, Chrétien backed off from saying that Rae is his No. 1 choice.

He said there are good candidates to choose from, and Rae is among them. "He's perfectly bilingual, he has a lot of experience, but they might for some reason not select him," Chrétien told reporters.

Other names that have come up as possible contenders for the interim and permanent leadership are Marc Garneau, Ralph Goodale, Dominic LeBlanc and Justin Trudeau.

Last week, Rae told CBC News he wouldn't be able to decide whether he was interested in being interim leader until he knew whether it would disqualify him from taking on the role more permanently.

"I can't say I'm a man without ambition, but at this stage in my life I am a realist about what can be done and what people are asking me to do," Rae said.

Chrétien repeatedly said the future leader, and the future of the Liberal Party itself, is up to the party to decide. The idea of a Liberal-NDP merger down the road is being discussed as a potential scenario in the wake of the decisive majority won by the Conservatives in the May 2 election and the NDP's new role in the House of Commons as the Official Opposition.

"It's not for me to decide," said Chrétien. "It is for the leadership of the party to decide. I don't know what they will do, it's up to them."

Chrétien recalled a conversation he had last year with former NDP leader Ed Broadbent on the idea of a merger of their two parties.

"But we were there as elder statesmen in a way, we had no executive power. And we stopped talking when my leader said, 'I'm not interested,'" Chrétien said.

With files from The Canadian Press