U.S. House Ways and Means Committee Chairman Richard Neal is moving forward with plans to subpoena President Donald Trump’s tax returns.

The Massachusetts Democrat is in the process of consulting with lawyers from the U.S. House and the Joint Committee on Taxation “to determine the appropriate legal steps to go forward with this unprecedented request,” William Tranghese, an aide to Neal, said in an email Monday.

He added that “a strong case is being built,” and Neal “will continue to conduct this process in a judicious, methodical and deliberative manner.” He also said Neal believes special counsel Robert Mueller’s investigation should be allowed to continue.

The comments come as billionaire investor and Democratic activist Tom Steyer purchased television ads aimed at pressuring Neal to subpoena Trump’s tax returns.

The 30-second ad by Steyer’s group, Need to Impeach, urges Neal’s constituents to press him to subpoena the returns and vote to start impeachment hearings in the Democratic-controlled House. The group also plans to hold a town hall in Neal’s hometown of Springfield on Feb. 12 to talk about the urgency of beginning impeachment efforts.

“The American people need to know what Trump is hiding and Neal has the power to help uncover the truth,” Steyer said in a statement.

The Democrat had earlier said he plans to ask for Trump’s tax returns but also hoped Trump would release them voluntarily. Trump broke with decades of precedent by presidential candidates and refused to release his tax returns during his 2016 campaign. He said he was being audited, even though IRS officials have said taxpayers under audit are free to release their returns.

Other liberal and Democratic-leaning advocacy groups have also begun pressuring Neal to immediately begin the process of obtaining Trump’s personal and business tax returns.

The groups Tax March, Stand Up America, and Indivisible last month sent Neal a letter urging him to immediately seek the documents. They also drafted a letter that they say Neal could send to Treasury Secretary Steven Mnuchin and IRS Commissioner Charles P. Rettig to request Trump’s returns.

(AP)