The seat in Newfoundland and Labrador was vacated by former cabinet minister Peter Penashue last month, after he acknowledged his campaign received ineligible donations in the 2011 general election.
Penashue paid back $48,000 to the Receiver General of Canada, and blamed the problems on an inexperienced campaign volunteer.
Penashue is running for the seat again, with Harper's full backing.
He will be competing against Liberal Yvonne Jones, a former member of the provincial legislature, and Harry Borlase of the NDP, a researcher and analyst on northern issues.
Penashue defeated the Liberals in 2011 by just 79 votes.
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