Trump, taking to Twitter after Nadler's comments, lashed out anew at "more than two years of Presidential Harassment."
"I am an innocent man being persecuted by some very bad, conflicted & corrupt people in a Witch Hunt that is illegal & should never have been allowed to start," he wrote. "And only because I won the Election! Despite this, great success!
Several other House committees are digging aggressively into the President, his 2016 campaign, his businesses and his associates.
Nadler's announcement came just days after the House Oversight and Reform Committee publicly questioned former Trump personal lawyer Michael Cohen, who implicated Trump in several serious crimes, including potential campaign finance violations connected to hush-money payoffs to women and possible fraud charges concerning falsified documents provided to banks and insurance companies.
Representative Elijah Cummings, D, the oversight panel's chairman, said last week that he, too, was interested in securing testimony from Trump's children and Weisselberg, who has overseen the financial details of Trump's business ventures for decades.
Also last week, House Ways and Means Committee Chairman Richard Neal, D, asked his staff to begin preparing a request for Trump's past tax returns to be submitted to the Treasury Department in the coming weeks. The request, first reported by NBC News, was confirmed by two Democratic aides familiar with Neal's action but not authorised to discuss it publicly.
The House intelligence and financial services committees also are amid probes that could touch Trump personally.
Nadler's probe, however, is unique: Only the Judiciary Committee can recommend the President's impeachment - a politically explosive move that has been handled carefully by House Democratic leaders even as more and more rank-and-file Democrats say Trump has committed impeachable offences.
A person who was familiar with the pending document requests but was not authorised to comment publicly on the matter said requests dealing with potential obstruction of justice would focus on Trump's alleged efforts to remove perceived enemies at the Justice Department, including former FBI Director James Comey, and install more loyal replacements.
The requests would also look at potential abuses of power, the person said, including the possible dangling of pardons and witness tampering, as well as Trump's broader attacks on the entities investigating him and the press.
A spokesman for the Judiciary Committee declined to comment.
Nadler said he has made no determination on whether to proceed with impeachment. But he said he was personally convinced that Trump has obstructed justice - an offence that was included in the impeachment articles passed in the House against both Richard Nixon and Bill Clinton.
The Judiciary Committee probe, as described by Nadler, would supplement the investigation by Special Counsel Robert Mueller focused on ties between Russia and Trump's business and campaign. Mueller is widely thought to be in the closing stages of his investigation, with Justice Department officials expecting to receive his report by the end of the month.
"This investigation goes far beyond collusion - we've seen all the democratic norms that we depend on for democratic government attacked by the Administration," Nadler said of his own probe, pointing to Trump's attacks on the press, intelligence agencies and federal law enforcement.
"All of these are very corrosive to liberty and to the proper functioning of government and to our constitutional system. All this has to be looked at and the facts laid out to the American people."
House Speaker Nancy Pelosi, D, said that she would await Mueller's findings, as well as a Justice Department investigation based in New York that is probing allegations that Trump paid hush money to two women to conceal affairs, before determining whether the President should be impeached.
"Impeachment is a divisive issue in our country, and let us see what the facts are, what the law is, and what the behaviour is of the President," Pelosi said. "When the facts are known, then we'll make a judgment then."
But Cohen's testimony has further emboldened rank-and-file Democrats, who are seeing fewer and fewer reasons to shy away from discussing impeachment.
Congressman Daniel Kildee, D, said in a CNN interview that Cohen "put some information into public circulation that's pretty damning to the President."
"If we find facts that clearly impugn the integrity of the president and potentially reveal crimes, we just don't have a choice," he said. "The Constitution is clear: Congress has to do its duty."
Republicans have amped up the threat of Democratic impeachment proceedings - believing that any attempt to remove Trump from office will motivate the Republican base and turn off swing voters.
Responding to Nadler, House Minority Leader Kevin McCarthy, R, said the House Democrats' investigations represented an effort to leave a cloud over Trump even after Mueller's findings are released.
"There's no collusion, so they want to build something else," McCarthy said, adding that Nadler "decided to impeach the President the day the President won the election."
Congressman Douglas Collins of Georgia, the top Republican on the Judiciary Committee, said he and his staff received no advance notice of the document requests, learning the news from Nadler's TV appearance. 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.
"Now that we seem to be getting closer to Mueller's (report) coming out, they're backing off 'collusion' and trying to find anything else that will stick," Collins said. "I think the story here is that Democrats are just trying to find anything to make the President look bad."
Nadler said that he was mindful of the political necessity of building a public case for impeachment - one that won't convince Democrats alone.
"Before you impeach somebody, you have to persuade the American public that it ought to happen. You have to persuade enough of the opposition party voters, the Trump voters, that you're not just trying ... to reverse the results of the last election," he said. "We may or may not get there. But what we have to do is protect the rule of law."
