BETHLEHEM, Pa. — Bob Brooks kept his message to the point in a six-minute address to union leaders and supporters on Monday. Brooks, the head of the state firefighters union and a Democratic candidate for a battleground House seat in the Lehigh Valley, described himself as a “working-class candidate” and “working-class person” who has been working since he ran a paper route at age 10. “The whole system is rigged against us,” he said — and the only way to combat such an imbalance of power is by “