WASHINGTON — Democrats have officially launched their assault on President Donald Trump's tax returns.

Ways and Means Committee Chairman Richard Neal, D-Mass., on Wednesday evening said he had filed a formal request with the Treasury Department for the documents.

“I today submitted to IRS Commissioner Rettig my request for six years of the president’s personal tax returns as well as the returns for some of his business entities. We have completed the necessary groundwork for a request of this magnitude and I am certain we are within our legitimate legislative, legal, and oversight rights,” Neal said in a statement.

It’s just the opening salvo in what’s sure to be a prolonged battle with the administration to see returns the president has refused to release.

Trump, who was meeting with senior military officials in the Cabinet Room, seemed unimpressed when he was told about the request for six years of returns by a reporter.

"Is that all? Usually it's 10, so I guess they're giving up," Trump quipped, before maintaining that he was still "under audit."

"I'm always under audit, it seems," he said. "Until such time as I'm not under audit, I would not be inclined to do that."

Treasury Secretary Steve Mnuchin said at a Ways and Means hearing last month that he would protect the president’s privacy if members of Congress tried to request his tax returns.

“I have discussed with the legal department in the Treasury that we will most likely receive this request. As I have said, based upon the request we'll examine it and we will follow the law,” Mnuchin said then. “And we will protect the president as we would protect any individual taxpayer under their rights.”

Democrats have been demanding to see Trump’s tax returns since the 2016 presidential campaign, with Neal’s request just the first stage of what will likely be a lengthy legal battle to obtain them. Since he was a candidate, Trump has claimed he can’t release them because he’s under audit by the IRS.

Ways and Means is one of three congressional committees that has the authority to seek to obtain the president's returns. The other two are the Senate Finance Committee and the Joint Committee on Taxation.

NBC News reported last month that Neal had asked his committee’s attorneys to prepare the request for the tax returns.

Senate Finance Committee Chairman Chuck Grassley, R-Iowa, told NBC News Wednesday that he believes the Democratic request for Trump’s tax returns is “political,” and that while congressional oversight is legitimate, “doing it for political purposes is not legitimate.”

In early March, Grassley had told reporters if House Democrats were to request the returns, he expected his committee would do the same, but said that they’d remain private required by law. Grassley, however, said Wednesday he doesn’t think he will also request his returns in response to Neal's move.

Speaker Nancy Pelosi, D-Calif., has repeatedly reiterated that it is up to the Ways and Means panel to decide whether to request Trump’s returns.

As the committee's chairman, Neal has the power to send a written request to the IRS to provide the information. If the Treasury Department were to deny it, House Democrats will have to decide whether to pursue the tax returns through a legal route. If they are obtained, Neal would then have to designate the panel’s members as “agents” to read the returns. They would then have to vote to make the documents public and report them to the full House.