President Donald Trump asserted executive privilege on Wednesday to keep under wraps documents on adding a citizenship question to the 2020 U.S. census, defying a House panel's subpoena in another move to stonewall Democratic lawmakers' investigations.

Despite Trump's assertion of the legal doctrine, the House of Representatives Committee on Oversight and Reform pressed forward with its plans to vote on holding two members of Trump's Cabinet — Commerce Secretary Wilbur Ross and Attorney General William Barr — in contempt of Congress over the census matter.

The fight over adding a citizenship question to the census presents high stakes for both Republicans and Democrats, with the 2020 U.S. elections looming. The U.S. Supreme Court is due to rule by the end of this month in the Trump administration's appeal of a judge's ruling blocking the addition of the question as a violation of federal law.