John Manley has announced his intention not to run for the leadership of the Liberal Party, but the former deputy prime minister and finance minister urged his party to act responsibly in its new role.

"I know the commitment involved both in a leadership campaign and then if you win, in rebuilding the party, in seeking election," Manley said on CTV's Mike Duffy Live. "And if successful, the prize at the end of the day is a lot of hard work."

"I was ready for that three years ago when I sought the post, but you know times change and life moves on. It may not be forever but right now it wasn't the time to go back," Manley said.

He made his decision public in a column in Thursday's Globe and Mail, and confirmed it on Mike Duffy Live.

Manley also said it would be a mistake for the Liberals to try and bring down the new government. He said time spent on the opposition side of the House of Commons will be good for the party and good for Canada.

"A party in opposition has time to renew its thinking on issues that perhaps its mind have been somewhat closed on," Manley said, noting that the aging of the population and the divide between cities and rural areas are issues that the Liberals need to take a fresh look at.

He cautioned the Liberal Party to accept the role of opposition and respect the new prime minister.

"He was elected to govern and they should deal responsibly with the challenge of being in opposition, a loyal opposition in a parliamentary system which challenges the government but essentially allows the government to do what it was elected to do, which is to make the crucial decisions on behalf of the country."

Manley pointed out that the opposition will present a formidable challenge to the new government, with many former ministers who are intimately familiar with files that are new to the new government.

The top Liberal post is up for grabs after Paul Martin announced he would step aside Monday night after election results confirmed the Conservatives led by Stephen Harper would form a minority government.

Manley has retired from public life and is currently with the law firm of McCarthy Tetrault as senior counsel. A source told The Canadian Press that the former deputy PM is happy there.

Manley ran for the leadership of the party in 2003, but pulled out early after it looked certain that Martin was headed for landslide victory. After winning, Martin offered Manley the ambassadorship to Washington. He turned it down.

Instead, Frank McKenna took the post -- and now he himself is considered a potential frontrunner in the race to replace Martin.

Speculation about McKenna's political intentions were fed after he resigned as ambassador. He informed Harper of his decision on Tuesday, Canadian Embassy officials in Washington said Wednesday.

In his two-page letter to Harper, McKenna asked to be relieved of his duty, adding he shared a mutual commitment to Canada with Harper and that he would have had no difficulty working with a Conservative government.

However, "he believes that it would compromise the position of the ambassadorship" were he to stay on, CTV's Joy Malbon -- who obtained a copy of the letter -- said on Newsnet's Mike Duffy Live.

McKenna has not publicly stated whether he will run.

Others named as potential candidates for the Liberal leadership include recently elected Liberal MP Michael Ignatieff. The Harvard academic has long been touted as a potential future party leader.

Former cabinet ministers Scott Brison and Belinda Stronach, and former Newfoundland premier Brian Tobin, have also been mentioned by pundits as possible contenders.

It's not clear when the leadership convention will be held. CP reports that party leaders suggest sometime between November 2006 and March 2007.