WASHINGTON — A powerful Democratic House committee chairman investigating possible abuses of the government’s security clearance process stepped up demands on Friday to see key documents and interview potential witnesses from the White House in light of a new report that President Trump personally intervened to grant his son-in-law a top-secret clearance despite legal and national security concerns.

The chairman, Representative Elijah E. Cummings of Maryland, who leads the House Oversight and Reform Committee, accused the White House in a new letter of stonewalling his requests for information and implied that if it did not comply voluntarily, he would issue a subpoena to compel its cooperation.

He said the report, published by The New York Times, added new concerns that Mr. Trump was lying to the public about his role in the clearance process to existing and broader questions about irregularities surrounding who should have access to sensitive government secrets.

“If true, these new reports raise grave questions about what derogatory information career officials obtained about Mr. Kushner to recommend denying him access to our nation’s most sensitive secrets,” Mr. Cummings wrote in a letter to Pat Cipollone, the White House counsel. The letter went on to ask about “why President Trump concealed his role in overruling that recommendation, why General Kelly and Mr. McGahn both felt compelled to document these actions, and why your office is continuing to withhold key documents and witnesses from this Committee.”