He earns very little money or uses loopholes and tax shelters and, as a result, pays little or no tax.
Many voters were shocked when Mitt Romney, the 2012 Republican presidential candidate, disclosed that he paid an average of just 14 percent of his income in taxes. Mr. Trump might pay even less, perhaps even zero. Real estate developers like Mr. Trump can take advantage of numerous deductions, credits and other tax loopholes that allow them to minimize how much tax they owe. In fact, Mr. Trump has boasted about avoiding taxes, telling George Stephanopoulos of ABC in May, “I fight very hard to pay as little tax as possible.”

Tax records from New York City also show that Mr. Trump has, as recently as this year, received a property tax credit that is available only to families earning less than $500,000 a year. His campaign has said that the credit was issued in error. But he has received the same credit in the past. It also appears that Mr. Trump paid little or no personal income taxes in some years in the early 1990s and late 1970s, according to news accounts based on documents from gambling regulators in New Jersey.

He gives little or nothing to charity.
Mr. Trump’s campaign claims that he has donated tens of millions of dollars to good causes. His last donation to the Donald J. Trump Foundation, which he and his family control, was made in 2008, according to The Washington Post, and there is no evidence that he has given much to other charities. On Tuesday, The Washington Post reported that his foundation spent $293,000 to settle lawsuits involving Mr. Trump’s businesses, pay for advertising for his hotels and buy two portraits of himself, in apparent violation of tax laws that prohibit the use of tax-exempt funds for the benefit of its executives and their businesses. That hardly inspires confidence.

His tax returns could shed light on whom he does business with.
The detailed schedules in his tax return could reveal the source of his earnings and debt. This would provide information about his ties to Russian oligarchs, Middle Eastern billionaires and other figures closely linked to foreign governments that might be trying to influence or thwart American policy. A recent Newsweek story revealed that Mr. Trump has done business with politically connected businessmen in Azerbaijan, Dubai, India, Turkey and elsewhere. And a few years ago, his son Donald Trump Jr. said that many Russian investors had put their money into the family’s business.