House Judiciary Committee Chairman Jerry Nadler said Friday that negotiations with former White House counsel Don McGahn over an outstanding subpoena for his testimony, but if the outstanding issues are not resolved soon, the committee plans to go to court to seek enforcement of the subpoena early next week.

Nadler told reporters at the Capitol that the committee would also be filing an application for the grand jury material underlying former special counsel Robert Mueller's report on Russian meddling in the 2016 presidential election.

"The Department of Justice policies will not allow prosecution of a siting president," Nadler said. "The House is the only institution that can now hold President Trump accountable for these actions. To do so, the House must have access to all the relevant facts."

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He added, "We will continue to seek testimony from key fact witnesses," and he added that "our work will continue into the August recess."

McGahn, who was prominently featured in the Mueller report, told the special counsel's team during hours of interviews that the president had ordered him to fire Mueller, and he refused. He has already defied a subpoena by the House panel, declining to testify when the Justice Department Office of Legal Counsel claimed executive privilege.

Asked about the growing calls among the Democratic caucus to file articles of impeachment for Mr. Trump, Nadler did not rule out the possibility.

"We may decide to recommend articles of impeachment at some point. We may not," he said. Several of Nadler's colleagues referred to their next step in their inquiry as an "impeachment investigation" into the administration.

While House Speaker Nancy Pelosi continues to resist calls for impeachment, Nadler said the caucus will "do what we have to" to hold the president accountable. Still, he echoed Pelosi's assessment that if the House brings articles of impeachment, "we must make the strongest possible case."

Rebecca Kaplan and Emily Tillett contributed reporting.