Mr. Brown, who has never held elected office, has found himself outpaced by Senator Jacky Rosen, the low-key and well-financed Democratic incumbent. A debate Thursday night presents a key test.

Fighting from behind in the polls, Brown was combative throughout the hour-long debate against Democratic Sen. Jacky Rosen.

Sam Brown, an Army veteran, went on offense in their debate, continuously painting the incumbent senator as an out-of-touch elite who violated rules on stock trading. However, Rosen wasn’t rattled and tried to pivot the conversation back to policy.