(CNN) The impeachment of President Donald Trump suddenly looks like much more than just a theoretical possibility.

Democrats on Monday will launch an "abuse of power" investigation that could be easily transformed into an even more serious process, with an expansive demand for documents from Trump's government, his family and even his real estate empire.

The President reacted to his worsening plight with a vehement defense on Sunday, after a week in which testimony from his ex-lawyer Michael Cohen deepened his political vulnerability and ahead of the expected filing soon of special counsel Robert Mueller's report.

"Presidential Harassment by 'crazed' Democrats at the highest level in the history of our Country. Likewise, the most vicious and corrupt Mainstream Media that any president has ever had to endure," Trump tweeted Sunday night.

Presidential Harassment by "crazed" Democrats at the highest level in the history of our Country. Likewise, the most vicious and corrupt Mainstream Media that any president has ever had to endure - Yet the most successful first two years for any

House Judiciary Committee Chairman Jerry Nadler, who would eventually lead any impeachment proceedings, on Sunday signaled a significant escalation into congressional inquiries into the President.

The New York Democrat plans on Monday to request documents from 60 people and entities close to Trump, including from the Department of Justice, the White House and the Trump Organization.

The document trawl will be used "to present the case to the American people about obstruction of justice, about corruption and abuse of power," Nadler said on ABC News' "This Week" on Sunday.

Nadler stuck to the House Democratic position that impeachment "is a long way down the road," apparently in order to avoid Republican arguments that the decision has already been made to try to oust Trump. The document requests are not taking place under the auspices of an official impeachment investigation.

But Nadler said nevertheless that he believes the President had obstructed justice, a potentially impeachable offense.

And given his responsibilities and powers, the warning from Nadler took the President's political and legal nightmare to a new plane, and opened a new, more serious stage of the showdown between House Democrats and Trump.

It was the latest sign that investigations sparked by accusations that Trump's campaign team cooperated with Russian election meddling has mushroomed into a relentless examination of Trump's political, personal and business life.

The latest blow to the President further intensified pressure on the White House as Washington waits for another shoe to drop — with Mueller expected to file his long awaited report from Monday onward.

The Nadler investigation, along with parallel probes into Trump's presidency by the House Oversight and Intelligence committees means that the structure of a political investigation into the President based in Congress is now in place, alongside the legal inquiries led by Mueller and prosecutors in New York and other jurisdictions.

Trump showing signs of stress

In the last week, partly through Cohen's testimony, it has become clear that even if the special counsel does not find direct wrongdoing by the President, Trump's legal troubles will linger deep into his presidency and probably beyond.

Trump is showing increasing signs of stress at being surrounded.

He spent much of the weekend laying out a likely defense should any of the multiple probes find him guilty of wrongdoing and bolstering his stranglehold on the Republican Party that could eventually be key to saving his presidency in a Senate trial if House Democrats opt for impeachment.

"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 - And only because I won the Election!" Trump tweeted Sunday

In a mostly unscripted two-hour speech to the Conservative Political Action Conference on Saturday — one of the most demagogic and inflammatory appearances of his presidency, Trump lacerated Mueller and his investigation

"So now we're waiting for a report, and we'll find out ... who we're dealing with ... We're waiting for a report by people that weren't elected," Trump said at CPAC.

"You put the wrong people in a couple of positions and they leave people for a long time that shouldn't be there, and all of a sudden, they're trying to take you out with bullshit, OK," Trump said.

Trump also sketched a defense for two potential areas of vulnerability: his call for Russia to find Hillary Clinton's missing emails during the 2016 campaign and his firing of former FBI Director James Comey in 2017.

He said he was being "sarcastic" when he asked Russia to find Clinton's emails and was having fun with his audience.

Mueller's team has already filed an indictment against 12 Russian intelligence operatives, accusing them of hacking into Clinton's personal emails for the first time on the same day — July 27, 2016 — as Trump's appeal.

Trump's comment is often cited by his critics as an instance in which his campaign colluded in plain sight with the Russian election meddling effort.

The President also used his speech at CPAC to knock back accusations that his firing of Comey was an attempt to shut down the Russia investigation and therefore fits the definition of obstruction of justice.

"I said, 'Melania, I'm doing something today, I'm doing it because it really has to be done ... he's a bad, bad guy,'" Trump said, arguing that he thought Democrats would welcome the move given their anger at Comey's handling of the Clinton email investigation.

"I said to the first lady, I said, 'but you know the good news, the good news is that this is going to be so bipartisan, everyone's going to love it' — so we fired Comey."

In May 2017, Trump told NBC News that he was thinking of the "Russia thing" when he dismissed Comey.

His lawyers have argued that since he is the titular head of the US government and legal system, the President has the right to dismiss anyone in the executive branch and therefore cannot be guilty of obstruction.

It may soon be Mueller time

Mueller has not so far produced any evidence that Trump is guilty of cooperating with the Russian election interference effort, or of obstructing justice.

He has however sprinkled tantalizing clues in indictments of a handful of Trump associates that have sparked intrigue about what his eventual findings — that will be presented to Attorney General William Barr — will show.

In a moment that will have dramatic overtones given the timing, Barr is due to deliver brief remarks on Monday at an event at the White House hosted by the President for state attorneys general, a Justice Department spokesman said.

Trump's fierce political campaign against what he calls Mueller's "Witch Hunt" and "hoax" investigation is apparently aimed at discrediting any conclusions that Barr choses to share with Congress and the public.

But his increasingly emotional denunciation of the various legal and political investigations — that are now focusing on his business, his campaign, his transition, his inauguration and his presidency is not giving the impression that he is a President who is confident there will be no charges to answer.

Nadler is expected to give further details of his document request on Monday.

But he said on ABC that it would stretch from the White House, to the Department of Justice to Trump's son, Donald Jr., and Allen Weisselberg, the chief financial officer of the Trump Organization.

Given that he believes that Trump obstructed justice, Nadler was asked on "This Week" whether the decision not to pursue a formal impeachment investigation at this point was merely a political distinction.

"We do not now have the evidence sorted out and everything to do an impeachment. 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," Nadler said.

Republicans accused Nadler and fellow Democrats of lining up a fall back investigation to pursue the President in case Mueller does not find offenses that rise to the level of impeachment.

"I think Congressman Nadler decided to impeach the President the day the President won the election," House Minority Leader Kevin McCarthy said on "This Week."

"He talks about impeachment before he even became chairman and then he says, you've got to persuade people to get there."

McCarthy also argued that the hush money payments made by Trump to two women who accused him of affairs before the election did not amount to the standard of impeachable offenses.

Cohen last week produced a check for $35,000 which he said was proof that Trump was reimbursing him for what may amount to an infringement of campaign finance laws even while he was in office.

But McCarthy argued that campaign finance violations merit a fine, not the ultimate sanction Congress can take against a President.

"Those aren't impeachable in the process," he said.