Trump’s decision on whether or not to release his tax returns is personal not legal. There is no requirement that a president must release his taxes, but there is also no law or regulation against releasing taxes that are under audit. Although the IRS routinely audits a president’s tax returns each year; all prior presidents for more than three decades have made public their returns. For Trump, either decision on his taxes could contribute to his impeachment.

Trump’s refusal to release his tax returns creates the plausible inference that he has something to conceal from Congress and the American people. What matters for impeachment is what precisely he is hiding. Presume that the returns are only embarrassing politically, showing for example, that he paid little or no federal income taxes in some years. The wise course for the president would then be to release his returns, otherwise his credibility will suffer and impede his defense against any potential charges of impeachment.

During the campaign, Trump handily dismissed charges that he evaded taxes, even bragging that paying no tax “makes me smart.” He could survive similar charges as president, while clearing the air of suspicions about more serious revelations. And Trump would gain political capital for openness and transparency. Thousands of Americans recently took to the streets to demand that Trump release his tax returns. Polls show that more than two-thirds of the public agree that Trump should release his returns.

Presume, however, that the returns contain explosive information relevant to charges of impeachment. For example, his tax documents could reveal undisclosed financial ties to foreign governments or foreign controlled banks and corporations, leading to charges that he is violating the Emoluments Clause of the Constitution. This provision prohibits all federal officials, including the president, from receiving anything of value from foreign entities. The prohibition is absolute; no amount is specified, and a quid pro quo is not required to trigger a violation. Impeachment is the remedy for violating the Constitution.

The tax returns could conceivably reveal financial obligations to Russian oligarchs or Russian banks, even indirectly through companies in which Trump has a financial stake. Trump has said, “I have no business there and no loans from Russia.” But in 2008 his son Donald Jr. said, “Russians make up a pretty disproportionate cross-section of a lot of our assets; say in Dubai, and certainly with our project in SoHo and anywhere in New York. We see a lot of money pouring in from Russia.”

If his tax returns contain such explosive information, Trump will likely decide to continue stonewalling in the hope that his taxes remain secret. However, the FBI and two congressional committees are investigating the possible involvement of Trump and his associates with Russia’s assault on our democracy during the presidential election. The FBI or either committee may decide to subpoena Trump’s tax returns as relevant to their investigation.

Trump could resist a subpoena, but would then find himself caught in the Richard Nixon impeachment trap. In 1974, the House Judiciary Committees voted for an article of impeachment which said that Nixon had “failed without lawful cause or excuse to produce papers and things as directed by duly authorized subpoenas issued by the Committee on the Judiciary of the House of Representatives.”

The Article charged that the president was illicitly “substituting his judgment as to what materials were necessary for the inquiry, interposed the powers of the Presidency against the lawful subpoenas of the House of Representatives.” Among the many parallels between Trump and Nixon that I analyze in The Case for Impeachment, Trump has claimed absolute presidential authority and shown little regard for the constitutional separation of powers, as evidenced by his defense of his initial travel ban.

Trump’s ongoing defiance of public opinion in concealing his tax returns may only reflect misguided political judgment. Or it may be a vain effort to conceal information that could contribute to his impeachment.

Allan J. Lichtman is Distinguished Professor of History at American University and author if The Case for Impeachment (Dey Street Books, April 2017)