At the convention, a candidate needs 60 percent of the delegate vote at any point to secure the nomination, otherwise, the top two finishers advance to a primary election. So if Herbert isn't eliminated at the convention, and doesn't clinch the nod there, he'll face one primary opponent.
He could still be booted at the convention. In 2010, then-Sen. Bob Bennett failed there (Kirkham worked to help unseat Bennett) amid a multi-candidate field to advance to a primary that Republican Mike Lee eventually won.
Kirkham is a household name in Utah tea party circles and would likely be able to coalesce much of the grassroots support on the right. That said, the presence of two other challengers in the race would complicate a Kirkham campaign.