Hillary Clinton had been waiting for this one: a line of questioning about Donald J. Trump’s refusal to release his income tax returns.

And she pounced at Monday night’s debate, suggesting that perhaps Mr. Trump was concealing that he had not been so charitable or was not as wealthy as he claimed.

But another possibility she raised — that Mr. Trump had not been paying income taxes — set off a curious response from him that sounded a lot like an admission.

“That makes me smart,” Mr. Trump said after Mrs. Clinton brought up how he had paid no taxes more than two decades ago. When she suggested that Mr. Trump was still paying no federal taxes, and had not done so for many years, Mr. Trump offered another retort: “It would be squandered, too, believe me.”