House Democrats on April 3 formally demanded that the IRS turn over six years of personal and business tax returns for President Donald Trump, setting the stage for a clash with the White House and in the courts.

Ways and Means Chairman Richard Neal of Massachusetts sent a letter to Internal Revenue Service Commissioner Charles Rettig, citing in part Congress’s oversight responsibility to review the president’s business activities before and during his first term.

He said he wanted the returns by April 10, but that deadline was expected to be negotiable. Trump told reporters he would refuse the request. The...