Pressed by host George Stephanopoulos that obstruction of justice on President Trump's part would be grounds for impeachment, Nadler said his panel isn't ready to reach that conclusion yet.
Nadler went on to explain, "Eleven-hundred times [Trump] referred to the Mueller investigation as a witch hunt".
Mr Nadler's added lines of inquiry also come as special counsel Robert Mueller is believed to be wrapping up his work into possible questions of Trump campaign collusion and obstruction in Russia's interference in the 2016 presidential election.
- House Judiciary Committee chairman Jerry Nadler, on ABC's The Week.
Nadler also said the list would likely include former White House chief of staff John Kelly and former White House counsel Don McGahn. "We do not now have the evidence all sorted out and everything to do an impeachment", Nadler said.
"We will do everything we can to get that evidence".
"We do not now have the evidence all sorted out and everything to do an impeachment", Mr. Nadler, of NY, told ABC's "This Week". He said Republican lawmakers "threatened to impeach people in Justice Department, they threatened the - the Mueller investigation".
"Impeachment's a long way down the road", said Nadler, whose panel would begin any impeachment process. "And if you then say, because the president can not be indicted, you're not going to give the evidence of his crimes, if any, to the public, you're saying the president can not be held accountable".
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Still, Nadler expressed hesitation about about pursuing impeachment against the president, telling Tapper Democrats "don't have all the evidence" to justice that action.
He also said the Judiciary Committee will continue its investigation even if special counsel Robert Mueller's probe finds no evidence of collusion between Trump's presidential campaign and Russian Federation.
Testimony by Michael Cohen, Trump's longtime personal lawyer, to the House oversight committee last week yielded new leads for committee investigators to follow as they probe whether the president engaged in corruption, obstruction of justice and abuse of power.
Sen. Mark Warner of Virginia, the vice chairman of the Senate Intelligence Committee, told Chuck Todd of NBC's "Meet the Press" that he also had serious concerns about whether Trump had committed crimes.
A spokesman for the Judiciary Committee declined to comment. Collins, too, said it was noteworthy that Democrats are moving beyond the Mueller probe with their own investigations - suggesting, he said, that they will do anything to find grounds to impeach Trump. "You have to persuade enough of the - of the opposition party voters, Trump voters, that you're not just trying to..."
Despite Nadler's hesitation on impeachment, later in the show, House Minority Leader Rep. Kevin McCarthy (R-Calif.) charged, "I think Congressman Nadler made a decision to impeach the president the day the president won the election".
Several Democratic representatives have called for impeaching Trump, even introducing articles of impeachment on multiple occasions. We may or may not get there.