Representative Jerrold Nadler of New York, a Democrat whose district includes Lower Manhattan, has written a scathing open letter denouncing Donald J. Trump for receiving grant money intended for small-business owners after the Sept. 11 attacks.

Mr. Nadler’s broadside against Mr. Trump is the latest salvo in efforts by Democrats to illustrate his use of various loopholes over the years to help his businesses.

Mr. Trump, the presumptive Republican presidential nominee, frequently invoked New York’s post-9/11 resilience to combat criticism from his primary rivals over his New York roots.

Mr. Nadler described those words by Mr. Trump as “exploitation” of the city’s darkest day.

“When do you plan on returning the taxpayer money that was designated to ease the suffering of our city’s small-business owners?” Mr. Nadler wrote in the letter, a copy of which his office provided to The New York Times.