OTTAWA—Frustrated grassroots Conservatives are mounting efforts to wrest control of candidate nominations away from senior party officials, as members prepare for a national convention where Pierre Poilievre is set to face a critical test of his leadership.

The Conservative party’s national convention will be held at the end of January in Calgary, near the rural riding where Poilievre clinched his parliamentary comeback after the loss of his Ottawa-area seat and a bruising election defeat earlier this year.

That spring campaign saw the party subjected to internal criticisms over how senior officials cherry-picked a number of candidates; something that is permitted under the party’s constitution, but rankled members who felt the grassroots — and prospective candidates — were ignored, poor

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