The inquiry could eventually reveal if the administration failed to follow regular procedures after Congress requested the president's tax records.
The House Ways and Means Committee has what they call "evidence of possible misconduct," but that's all we know.
A whistleblower apparently contacted House Democrats this summer about potentially "inappropriate efforts to influence" an audit of the president.
The Treasury secretary previously said he'd be happy to give Congress details on the process.
A recent report could lead to the National Rifle Association losing its tax-exempt status.
The bill, which would let Congress view the returns, now heads to the state assembly. The state information would mirror much of the federal tax figures.
From 1985 to 1994, Donald Trump's financial figures show he had more than $1 billion in business losses, according to The New York Times.
The Trump administration quietly moved to veil identities of big-money contributors to groups like the NRA and the ACLU.
Republicans once sought community group ACORN's tax information using the same law Democrats now cite to see Donald Trump's tax returns.
Trump's plan is apparently to disobey the law and hope a friendly judge bails him out.
"The litany of red flags is just extraordinary,” a former IRS official told The Trace.
The attorney known for representing Stormy Daniels is caught up in a "tangled financial web of lies," federal agent Ryan Korner said Thursday.
A bill with widely backed reforms almost got snagged over a provision blocking the IRS from creating free electronic filing.
Consumer advocates say they like the bill -- except for one provision they call a giveaway to tax preparers.
The treasury secretary suggested Democrats are out of line in their request for Trump's personal information.