US House Democrats have sent their long-anticipated request to the Internal Revenue Service for President Donald Trump’s tax returns for the past six years.

The request was the opening salvo in what could prove an explosive battle between Democrats and the Trump administration over the president’s personal finances that is expected to wind up in the courts.

“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,” House Ways and Means Chairman Richard Neal, D-Mass., wrote in a statement.

“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 that the decision to request the returns was in the interest of ensuring “the accountability of our government and elected officials. To maintain trust in our democracy, the American people must be assured that their government is operating properly, as laws intend.”

Democrats have said they would use Trump’s tax returns to inform their investigations into allegations of corruption and wrongdoing in the administration and the president’s family business.

Treasury Secretary Steven Mnuchin told lawmakers last month that the Treasury will “follow the law” and review any request. Trump, who has long considered any examination of his personal finances a red line for investigators, is expected to aggressively challenge Democrats’ request in court.

Trump broke with modern precedent as a White House candidate in 2016 when he refused to release his returns, and has continued to do so while in office. Most presidents since Richard Nixon have voluntarily released their tax returns over the past 40 years, which is not required by law.

Trump has said he continues to be under audit from the Internal Revenue Service, which the agency will not confirm or deny.

Trump’s attorney Jay Sekulow, who worked at the IRS shortly after law school, argued that Democrats would have to demonstrate a “legitimate legislative purpose” for obtaining the president’s returns, and predicted the request “probably gets litigated” in an interview with ABC News podcast “The Investigation.” (ABC News)