The famous “Buckley Rule,” named after conservative giant William F. Buckley, dictates that the best course in primary elections is to choose the most conservative candidate who can win.

The rule is usually invoked to discourage those on the Right from going overboard on ideological purity. But on March 20, nearly 49 percent of Illinois Republican voters observed the Buckley rule in its other, rarer-but-equally-important sense. They refused to vote for a Republican governor who, based on his record after four years in office, was not conservative enough to be worth keeping around.