In the midst of the contentious 2009 gubernatorial race in New Jersey, the state's teachers union took a poll of its own members and found only a slight majority preferred the candidate the union had endorsed, Democratic incumbent Jon Corzine, over Republican challenger Chris Christie. The alarmed union, the New Jersey Education Association (NJEA), swung into action with a campaign that included phoning 100,000 of its members and urging them to vote for Mr. Corzine, according to union documents leaked last November to the Education Intelligence Agency, a watchdog web site.

Mr....