WASHINGTON — Former White House counsel Don McGahn defied a House Judiciary panel subpoena Tuesday for materials he provided special counsel Robert Mueller, raising the specter of a legal fight as President Donald Trump pushes back against Democratic-led investigations.McGahn’s refusal to provide the documents came at the instruction of the White House, which suggested they could be subject to executive privilege. Such a claim can shield some presidential material from disclosure.Trump has defied House Democrats since the release of Mueller’s report . Republicans have largely united behind the president, with Senate Majority Leader Mitch McConnell on Tuesday declaring “case closed” on Mueller’s Russia probe and potential obstruction by Trump. McConnell said Democrats are “grieving” the result.Mueller said he could not establish a criminal conspiracy between the Trump campaign and Russia, but did not reach a conclusion on whether Trump obstructed justice. Mueller didn’t charge Trump but wrote that he couldn’t exonerate him, either.Democrats say the case is anything but closed and are conducting their own review of Mueller’s investigation of Russian election interference . The Judiciary panel wants to speak to McGahn and review certain documents, in part because he was a vital witness for Mueller, recounting the president’s outrage over the investigation and his efforts to curtail it.