A Georgia Mystery: How Many Democrats Voted in the G.O.P. Primary?

One look at the results of Georgia’s primary election last week led many Republicans to believe it was the product of Democratic meddling. Former President Donald J. Trump’s recruited challengers lost in grand fashion in his most sought-after races: David Perdue was routed by Gov. Brian Kemp by more than 50 percentage points, while Representative Jody Hice fell to Secretary of State Brad Raffensperger by nearly 20.

Mr. Trump and his allies pointed to so-called Democratic crossover voters as the cause of their shellackings. In Georgia’s open primary system, Democrats and Republicans can vote...