The National Education Association, the nation’s largest union, is threatening to cut off health insurance to about 300 Washington, D.C.-based workers on Aug. 1 in an effort to end a bitter contract dispute. It’s a tactic some private employers have used as leverage against unionized workers that has drawn scrutiny from congressional Democrats and is prohibited for state employers in California. Experts on labor law say they’ve never seen a union make the move against its own workers. “This is like a man-bites-dog situation where the union is now in a position as the employer,” said Paul Clark…