David McIntosh has been fighting for economic freedom for 30 years, and he is convinced the battle has reached a hinge moment. “The stakes really are that high,” says the president of the Club for Growth. “If we don’t do this, it’s all at risk. This is the moment.”

By “this” he means denying Donald Trump the Republican presidential nomination. The club is one of America’s most effective free-enterprise advocacy groups, and for months it has tried to alert voters, often without much other support, to the risks of a Trump...