OTTAWA - Bob Rae is shelving his long-term leadership ambitions in order to become interim leader of the devastated federal Liberal party.

The Toronto MP wrote his caucus colleagues Thursday to advise he's willing to let his name stand for the position of interim leader.

And Rae acknowledged that means he'll have to give up thoughts of one day running to become the permanent successor to Michael Ignatieff, who resigned after leading the once-mighty party to a historic defeat in the May 2 election.

"I shall abide by any rules about the interim leadership, agreed to by the caucus and by the (the party's) board (of directors)," he says in the letter.

"I have made it a watchword of my time in public life to practise the politics of unity and principled compromise. I shall continue to do so."

The party's directors have the authority to appoint an interim leader, based on the recommendation of the parliamentary caucus which appears to overwhelmingly favour Rae.

The board served notice last week that the interim leader must be bilingual and must promise not to seek the permanent leadership or engage in any discussions about a possible merger with the NDP.

There remains some dissension over precluding the interim leader from seeking the permanent post. But party president Alf Apps, who's been consulting with caucus, riding presidents and other Liberals, has said there appears to be a broad consensus in favour of imposing the condition.

On Friday, the board is expected to issue final rules for the interim leadership and for delaying the vote for a permanent leader, most likely until the fall of 2012.

Should Rae be chosen and perform well as interim leader, it's conceivable Liberal party brass could eventually choose to relax the prohibition on running for the permanent leadership.

But sources close to Rae say he's genuinely relinquishing his long-term ambitions in order to focus on the immediate and urgent task of rebuilding the party.

Rae, a one-time NDP premier of Ontario, flatly told The Canadian Press: "I'm not planning to run for the long-term leadership."

Many Liberals believe the party will have to slowly rebuild its base through the next two elections before it can hope to be in position to contend for power again. At 62, Rae suggested he's simply too long in the tooth to take on that challenge.

"I think it's just being realistic. I mean the fact is we're down to 19 per cent (of the popular vote), we're down to 34 seats," he said in an interview.

A permanent leader has "got to be prepared to commit to a decade and it's a decade of work. I think that's obviously a factor in my decision, as it would be in anybody's decision."

In his letter to caucus members, Rae laid down a condition of his own. He said he's only interested in being interim leader if the vote for permanent leader is put off for 18 to 24 months.

Under the party's constitution, the leadership vote should be held in October. However, the party's board is proposing to hold a special "virtual" convention on June 18, at which delegates will be asked to amend the constitution to allow the leadership vote to be postponed for up to two years.

Apps has said a broad consensus has emerged in favour of choosing a permanent successor to Ignatieff in the fall of 2012.

Rae said the party must put aside its seemingly perpetual leadership politics if it is to recover from the May 2 thrashing.

"At some point, people are going to have to say, 'Is this rebuild serious?' Because if it is, then this exclusive focus on leadership politics has got to come to an end for a while."

Until now, Montreal MP Marc Garneau was the only Liberal to express interest in the interim leadership. Given Rae's decision, Garneau said he wants to consult with his supporters before determining whether he should withdraw from the race.

"Obviously, when someone like Bob Rae comes into the picture, that is pause for thought," Garneau told CBC-TV.

In his letter to caucus colleagues, Rae wrote: "After the worst election defeat in our history, it is vital that we come together as a party and engage directly with Canadians about what matters to them.

"The pursuit of social justice and a sustainable prosperity in a united Canada has to remain our focus. We cannot afford to get caught up in internal wrangling."

Rae said he's a "glass-half-full person" who believes the centrist party can come back from near oblivion, provided all Liberals pull together.

"There is a strong need for a party that is not caught in the trap of ideological excess. We need to use the talents of every member of the Liberal team right across the country."