Mr Mueller decided not to pursue an obstruction charge against Mr Trump in part because of the Department of Justice guidelines that prevent the indictment of a sitting president.
The result of the legal action to gain access to secret grand jury evidence rests on Judge Beryl Howell applying a precedent that dates back to the impeachment of president Nixon.
Release of Watergate evidence was deemed lawful in 1974 because it fell under an exception authorising disclosure of grand jury material “in connection with a judicial proceeding”.
On Sunday, Rep. Nadler rejected the suggestion House Democrats were running out of time to launch formal impeachment proceedings because of the forthcoming 2020 presidential election.
“We have to defend the Constitution against these kinds of unconstitutional and illegal deeds,” he said. “We have to do this, whatever time frame there is.”
Rep. Nadler did not address whether Mr Trump should be prosecuted for possible obstruction of justice or other alleged crimes after leaving office, but suggested he might be in favour.
“Anyone else who had done what he did would have been indicted on a charge of at least five different major crimes,” Rep. Nadler said.
“And a president who is immune from prosecution by virtue of the Justice Department… should be prosecuted after he leaves office. Or a last impeach him, if you can prove those crimes.”