Harcourt said Tuesday that the decision not to renew his membership came after a combination of mistakes made by the party and its leaders.
"You can't alienate the whole environmental community ... as they did coming out against the carbon tax, then pull an ugly, nasty leadership coup against a fine person like Carole James, and then blow an election with probably one of the stupidest political blunders of all time in British Columbia, coming out against Kinder Morgan."
NDP Leader Adrian Dix abruptly announced during last May's election that the party opposed the Kinder Morgan pipeline expansion, when he had said previously that he would wait for the approval process to finish before making a decision.
The flip-flop stunned both labour and investment factions in the province and was a turning point in the provincial election that saw Christy Clark's Liberals win a majority government.
Dix replaced Carole James as party leader in 2011 after she quit amid criticism from her own caucus.
Harcourt, 71, said he's been "disgusted and alienated" by the series of mistakes made by the party he once led and has now become an independent sustainability democrat.
He was first elected premier in November 1991 in a landslide victory over the Social Credit Party. He resigned in 1996 amid the so-called Bingogate scandal that involved money raised by charity bingo being funnelled to the party.
Harcourt supported Mike Farnworth during the last leadership search, but said he wouldn't be endorsing any NDP candidate in the leadership bid set for September.
Farnworth and John Horgan are the only declared candidates running for the party's leadership to replace Dix.
While Harcourt said Farnworth hadn't spoken to him in three years, he has had contact with Horgan and said he respected Horgan for the environmental work he's done in the past.
Harcourt said he can see a path for the party if it's led by a strong, effective leader who can create a vision and gain a team that's capable of governing the province.
Horgan said Harcourt's decision to leave the NDP is disappointing and an indication of the huge challenges the party faces to reconnect with residents.
"The message it sends to me is that I have a lot of work to do. I have a lot of work to do to convince Mike Harcourt and other New Democrats and all British Columbians that there's a better way for us to proceed (with) a balanced approach to the economy and the environment."
Horgan said he agreed with Harcourt that the NDP has lost its way on major issues.
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