WASHINGTON — The early days of the Trump administration have turned Washington into a land of leaks, with a national security adviser’s telephone calls, raw intelligence dossiers and confidential C.I.A. surveillance schemes spilling into the public. But the holy grail of leaks remains elusive: President Trump’s tax returns.

As taxpayers scramble to file their own returns and Republicans push to overhaul the tax code, attention has shifted back to Mr. Trump’s taxes and how he could benefit from the sweeping legislation he hopes to sign. Looking for the ultimate leak, opponents of Mr. Trump have wondered aloud how it could be that top intelligence agencies are suddenly fonts of classified information while the staid tax collector remains hermetically sealed.

“The American people deserve to know what conflicts of interest are hiding in Trump’s taxes,” said Delvone Michael, a member of the Tax March executive committee, which is organizing a mass demonstration to call for the release of the tax returns next month. “Every president in a generation has released his taxes. Trump must too.”

Mr. Trump broke with 40 years of tradition as a presidential candidate, declining to make his tax information public on the grounds that he was undergoing an I.R.S. audit. The government has made clear that is, in fact, no impediment to release, but since corporations like the Trump Organization are audited constantly, the excuse will not disappear, and he is not expected ever to release his returns willingly.