most pressure he s faced yet. for the republicans, tonight poses another crucial test. front-runner ben carson dismissing growing scrutiny over apparent des krepcys in his life story. will carson s rivals or even debate moderators grill him tonight? his chief opponent, donald trump, is already teeing off, focusing on carson s description of his violent youth. this is the only election in history where you re better off if you stab somebody, what are we coming to? reporter: carson s business manager tells nbc news the controversy has lit a fire under the usually calm candidate. i think finally he had an epiphany and realized that, look, i have got to be ready for these moments, bring it on, i m ready to fight. reporter: with the economy in focus tonight, trump s challenge to emphasize substance over style and show how his billionaire business savvy would translate to a trump presidency. marco rubio is gaining ground,