As leader of the Progressive Conservative party, he helped form the the current Conservative Party of Canada after the Progressive Conservatives merged with the Western-based Canadian Alliance in 2003.
The 54-year-old Nova Scotia native stepped down from politics in 2015 just before Harper lost to Justin Trudeau and the Liberal Party.
Conservative Party members will vote on a new leader on June 27 in Toronto.
Current Conservative leader Andrew Scheer stepped down last month after weeks of infighting and a disappointing performance in parliamentary elections in October. Trudeau won a second term despite losing the majority in Parliament.
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