BLACKWOOD, N.J. — Hillary Clinton on Wednesday mocked Donald J. Trump as evasive and secretive after he suggested that he would not release his tax returns before the November election, which would be a break with 40 years of political precedent.

But Mr. Trump quickly hit back, saying that he still intended to release his tax returns as soon as a federal audit was completed — and that Mrs. Clinton was hitting him out of desperation.

Mrs. Clinton, at a rally here to open her campaign for the New Jersey primary on June 7, had just begun attacking Mr. Trump’s proposed tax cuts for wealthy Americans when a man in the audience called out, “What about his tax returns?”

Mrs. Clinton, who often ignores catcalls, smiled and said, “We’ll get to that.”

Moments later she urged the man and other voters to keep asking to see Mr. Trump’s tax filings and indicated that her campaign would make them an issue in the general election, saying, “We’ll get around to that, too.”